I didn't manage to notice any mention of certain particularities of the Big Battles so here we go (apologies if I'm repeating another post).

There is no tanking in big battles. The mobs that attack pick a specific target for themselves when they assault, and you are not on the list of choices. It is either a rohirrim soldier, an obstacle (if the mob is a sapper type) or a commander (if the attacker is itself a commander). Aggro does not switch this target. Only if the list of primary objects is exhausted - that is, all the defenders at that defense zone have been killed - will they start targeting you. And since you are the only target left, and there's more mobs pouring in all the time, you will be overwhelmed very quickly. (The point being, you must keep the defenders alive.)

There is no CC either in big battles. Think of it as every single mob from trash upwards having a permanent CC immunity buff. The only things that slow them down are defensive works and Engineer traps.

DPS on the other hand is just as effective as ever, and in fact very desirable in certain situations in certain BBs as defenders will be extremly hard pressed to survive the enemy waves. For example, if in Helm's Dike you get the secondary quest to defend Helm Hammerhand's statue, certain types of mobs will start spawning (orc vandals, troll brutes and so on) whose primary target (see tanking above) is not the defensive line but rather the statue. This will cause them run straight through the line and go for the statue, which will pull rohirrim targeting them into a wild chase. This leaves less defenders to the line (sometimes just the commander, who never moves) against a static number of attackers, which means they get ganged on and die fast.

If you don't want merit loss for lots of rohirrim dying in the line of duty, you are going to need strong dps or healing. Period.

The Battles are still in balancing flux so things may chance between current beta (beta #5) and release, but: in wall defense instances (Deeping Wall and Hornburg) the best strategy still remains to ignore enemies and concentrate on cutting their grappling hooks and felling their ladders. This guarantees that instead of rohirrim defenders getting ganged on, it is the attackers that are at material disadvantage and die off quickly. Strategic healing by commanders or a healer ensures not a single defender dies. Of course you need to deal with enemy catapults with your own; they hurt so much that ignoring them is pretty much a guaranteed loss.

The prohibition on switching specs in combat has already been mentioned, so no more of that. I don't think equipment switch restrictions in combat got mentioned yet, but I'm actually not completely in clear about its current state so others should explain. Last I heard was, weapons could be switched in combat but nothing else. (Hearsay, did not test.)