TL;DR = "Hi, can we haz optchuns?

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Besides the usual statistical problems in informal surveys, the xp issue itself is complicated by multiple things--multiple inputs, multiple reasons, multiple options. (I also agree with others that neutral players most likely wouldn't be opposed, though I think a lot of people may not understand reasons for more xp control.)
I'm one of those who feels that leveling is too fast.

My comparison isn't against just WoW and other games, though. Here is my own breakdown for the xp/leveling discussion, as disorganized as it is.
(A) Multiple reasons for wanting to throttle down xp gain
(A1) Stay on-level with content (soloers)
This is the reason that I've seen brought up most often, I think. The crux of this "on-level with content" reasoning is the idea that you want a proper, more immersive challenge as designed by Turbine (a side-effect for crafters is out-leveling your crafting ability and crafted gear too quickly). But this reason is also one that seems to get a lot of "anti" sentiment as well. After all, you don't need to do all the quests in a zone, right? However, no one, not even the "hardcore soloer," is asking that Turbine lower xp gain for
everyone across the board. It's a matter of playstyle choice; moreover, anyone who feels this A1 camp is silly shouldn't ask for xp buffs in any form because that would be hypocritical.
(A2) Stay on-level with people (groupers)
A reason that doesn't seem discussed as much is the side-effect of grouping with people who invest different amounts of time in the game. I fall under this group. I'm fine with out-leveling content or skipping entire regions because, yes, I think replayability in any game is an important factor for retention. However, I usually game in MMOs with specific people--many players do, whether as husband-wife duos or larger kinships or whatnot.
Even though I solo pretty much 100% of the time now, I actually
prefer small fellowship content; under normal circumstances, I consider myself totally a group player* because playing group content on-level with friends new and old is tremendously satisfying and fun. I usually grouped with two other people; we managed to stay close in level up to 40-ish even though I have
a lot more time to play than them. I kept myself busy with crafting, primarily. But to complicate matters, since we wanted to stay on-level with each other, the entire group was limited to the lowest common denominator, which was the wife of the pair (which also meant the husband wouldn't play
at all unless she was online). Long story short, I eventually gave up trying to stay at-level with them for a variety of reasons/causes.
I'm sure there are folks who fall under both A1/A2 camps.
(A3) PvMP
(I don't remember why nor do I understand the mechanics here exactly, but someone in the skirmish xp buff thread mentioned how they can't and won't skirmish because they want to PvMP at a certain level or something.)
(B) Multiple inputs and types
(B1) Quests
I think Turbine did well here in that they really encourage people to do the quests, which are written well, can be creative (not always kill-quests or deliveries), and are often quite immersive with the lore. The majority of xp gain comes from quest turn-ins, and it should stay that way.
(B2) Skirmishes
Skirmishes not only give mob kill xp but additional xp for skirmish deeds and the daily quest (probably for encounters as well). The reason why I'm listing this separately from both quest/mob xp inputs is because I believe the skirmish mob xp is actually buffed compared to world mobs, even without the current temporary buff (I noticed I gained xp very quickly through skirmishes pre-25%-buff at a rate I expected to only see from turning in quests). Skirmishes are a special input also because they're viable alternatives to accessing any number of things from class quest items to rep items to crafting materials to Legendary stuff... A player who wants an item they cannot otherwise gain for whatever reason (e.g. full fellowship questing) can skirmish, but if said player also falls under camp A1/A2 above, that's a problem.
(B3) Mob kills
World mobs from regular questing/farming give xp, which is affected by rest xp (see below). This xp is only a fraction compared to an actual quest turn-in, in general, but situations like craft farming (hides) and kill deeds quickly accumulate xp gain.
(B4) Deeds
Finishing deeds is like finishing a quest with an automatic turn-in, thus netting a chunk of xp.
(B5) Legendary IXP
Legendary ixp is a different type from toon xp and can actually be toggled off per item. All mob kills that give toon xp (green and higher difficulty) will give some ixp, whereas quests normally do not give any ixp unless they explicitly state so. When quests do give ixp, it's a substantial chunk. The thing about ixp is that many players will naturally attempt to level the LI faster (get the DPS up, etc.), so in most cases, while the ixp system isn't directly linked to the toon xp system, leveling your LI through skirmishes or repeatable quests will automatically bump your toon xp at the same time.
(B6) PvMP
Uh, I don't know how this works, sorry...
(C) Multiple current bonuses
(C1) Rest xp or "blue bar" (Enhanced xp)
Rest xp begins to accumulate after you've been logged off the game for a certain amount of time (an hour?). I believe there's a maximum one-level cap, and there might be additional rest xp bonuses if you log off in a city (not sure). Rest xp increases all mob kill xp gain (double) but does not affect quest xp gains--i.e. turning in a quest will merely bump up the blue bar's position. This is a global bonus for everyone.
(C2) Destiny points for "blue bar" gain
You can invest destiny points, earned primarily by leveling or PvMP, to extend your blue bar for more mob kill xp (Accelerated Experience). Starting out at level 10, you start out with five Accelerated xp perks; this refreshes at a rate of one perk every 24 hours. Since this is already an in-game option, I feel it could be buffed for folks who want faster leveling instead of creating global xp buffs since there are no options to tone down xp gain.
(C3) Temporary state bonuses
Every so often, there are special weekend, holiday, etc. buffs that range from something like 3% to 25% (current temporary skirmish buff). I believe these only affect mob kills, not quests, but I could be wrong. These kinds of state buffs are universal for everyone, and I wish I could "click off" the buff (like you can click off/remove buffs in WoW).
(D) Multiple xp control options
(D1) Complete experience toggle
This is the type of complete on/off xp switch that exists in WoW. It shuts down xp gain from all sources. I don't know the exact details of the mechanic in WoW, but the main possible problem I can think of is if people forget to turn on the xp toggle and turn in a quest, since quest xp is the primary toon xp source in LotRO; however, while that situation itself is a legitimate concern, the corollary argument of "too many customer service inquiries" is a poor excuse, IMO. That's kind of insulting. Arguing about the mechanics themselves is much more fruitful. For example, with a complete off switch, Turbine could make it impossible to actually turn in quests without a Las Vegas-style confirmation. Or look at whatever WoW is doing and consider their success and how people like/use the option.
(D2) Partial experience toggles
An option is partial toggling, such as switches for only skirmishes (if I want to farm marks but not gain levels like a fiend) or all mob kills (if I want to farm hides or do a kill deed without gain). This would, however, be a bit more problematic to implement due to additional complexity.
(D3) Experience debuff/buff
Destiny points already exist to buff gain, so I don't see why this couldn't be buffed further to help people who really want faster xp gain (e.g. for alts). At the same time, however, I feel in fairness that an xp debuff should exist to counter that option. This may not fit with the destiny point setup, but there might be a way to create a decent debuff.
(D4) Confirmation bottlenecking
This would be regular xp gain, but actual leveling would require active confirmation instead of just automatic dinging. When I first heard about this idea, I wasn't sure if I liked it, but then I realized this is really perfect as I read more comments about it. As a passive "switch," this particular option I feel is most elegant, obvious, and simple, and no one can "accidentally" forget this compared to a toggle because you already earned the xp. It's beautiful. Out goes the whole "customer service incidents" argument as well as complexity. And, yes, from my amateur point of view, this seems like it wouldn't be too difficult to program and implement. I personally favor this idea most for practical reasons.
*

Originally Posted by
Trilwych
Huh. I realized there's a problem with this idea: farming xp. It would be dumb, but I suppose it would be possible under this basic system to stop leveling at 30, farm only skirmishes for a month straight, and then super-level yourself to cap after accruing all that xp. I suppose a tweak would be limiting such a confirmation to quest-based xp only, but that still gets into the problem of mob/skirmish/other toon xp inputs. For an A2 "stay on-level with people" player like myself, this would be completely pointless because I wouldn't normally quest too much by myself anyway, which leaves all the other "I'll do this on my own time" activities--same old xp gain issue. Or the confirmation system could be applied in reverse, working on all toon xp inputs besides quest xp. Or, perhaps most easily, there could be a cap on stored xp in such a confirmation system.
All these multiple issues do complicate matters in terms of xp control, but I think it would still be worthwhile in terms of player choices and netting an even wider range of playstyle preferences.
The way traits and gear are done in this game, though, trying to scale the player toon to content as suggested previously would likely be a mess (what would you do with a LI, anyway?). But looking at skirmishes, it would be much easier for Turbine to scale content to level instead of idiosyncratic toons...
(D5) Experience conversion
It would be very interesting in terms of personalization to be able to convert toon xp to ixp or destiny points or some other currency, but I'll bet the programming would be an absolute bear. It would be like taking xp point gain and distributing among things besides toon xp, like legendary point distribution among legacies. But while it's a nice idea, I think it would be way too complicated to make happen (unless it's a pure one-way conversion, like toon xp to destiny points).
(D6) Mentoring
This would be a highly social resolution to on-level questing and whatnot. In an ideal world, having such a system would promote more player interaction, which is good in all sorts of ways from the "MMOs should be social games" viewpoint, but as Beldacar commented, doesn't necessarily help solo players (and nothing against them since I'm one now). And there would need to be decent rewards. Overall, from a practical standpoint, as wonderful as this type of option might be, it's probably far too complex a system to progam and instate into a game of this maturity. (Or maybe someone can prove me wrong there.)
(D7) Alts
As far as I'm concerned, this isn't a true option for controlling xp gain. If you're trying to level an LI, for example, it's tied to that toon so you're going to have to do stuff (e.g. skirmish) for runes regardless of toon xp gain; this would be problematic for A1/A2 folks. Also, if someone--mostly the soloer--feels leveling occurs too quickly on one toon, I don't see how it wouldn't be too fast on another toon and another. And eventually you'd run out of alt slots. Now, from a group perspective, if you're maintaining one toon for a group and playing a different toon at all "off times," then yes, alt-ing could be viable. But some people like myself prefer to group with our "mains" and leave alts for other things. In any case, this is a playstyle call as well.