I DMed a high level one-shot 2E game with 15 players. I was fairly new to 2E, but very familiar with the 1E adventure I was using. One guy convinced me to play a psionist from the new book, and I said fine... without actually looking up the rules for it. About a half hour into the game, said player decided it was time to kill the rest of the party. The next hour was spent running the combat where the psionist brutally killed the party using death screen (or something like that) and defended by inertial barrier (or something like that). The rest of the night was spent bitching about the player who lorded it over everyone else. I'm honestly surprised no one kicked his @$$. I've never allowed a psionist in any game I ran again.
As a player, I was in a 3E game with 9 players, 6 of which were regular. The other three were hit or miss, but they always came together or not at all. It was a strange setup, because 3 of us were mostly interested in the story and RP, while the other 3 were only interested in combat (one had a Nintendo hand-held he played on until combat started). Those were the 6 regulars, with the 3 irregulars usually enjoying a bit of both. The DM (best I've ever played under, and still one of my DMs) did an amazing job balancing story and combat, since he knew about the great divide between the table.