Not sure why you wanted to provoke me into bring up another game but...

Andy, Andy, Andy...

You do realize that the cap of a battlegroup (think raid size cap) in Dark Age of Camelot has always been 200 players. There are 3 realms. Any time a zerg starts against a zerg in Dark Age of Camelot, half a raid vs half a raid (with a potential of being aj'd by another half raid) in the frontiers, which usually is a relic raid is 200 with a potential to grow to much more than that. The first clustering of servers, 4 servers to be exact, (circa 2006) the level 20 to 24 rvr area saw 3 battlegroups of 100+ from about noon pacific to 10pm pacific. I was there. It was glorious. It was a slide show, though.

But... 300 people in one zone is not an MMO first. It is not the only time in an mmo that it has happened. Relic raids in 2005 to 2006 regularly had 200 to 300+ in one zone. It was workable until the hib animists decided to drop a power font and spam a keep with FnF turrets. which means the green realm had the ability with 3 characters to drop 100 pets each in a particular ground target and crash the zone.

Rocking the cap in Osgiliath is great but its not an MMO exclusive.