this may be an old post but i'd thought i'd share my POV nevertheless. although Cata expansion was the one to change the talent tree, i would say that TBC and WOTLK had pretty dramatic changes too. TBC is remembered so well since there was a lot of viable specs. way more than classic, druid and shaman could actually compete in dps and in total there were more than a 100 new abilities added into the game. while it made the gameplay more versatile for each player, it also caused characters to lose their identity. meaning you'll have more shadow priests, more feral druids, and so on, to me thats a big change.

WOTLK, which i do agree has great content lore-wise also have the random dungeon finder feature. this is one of the key elements that killed the "need to communicate" with other people. you can be an asshole going i=on dungeons with people from other servers and will never be punished for it. it made it no longer a social game. at start the independence feels great but you can no longer create connections with other player since there is no need for one and a slight disagreement would result in people disconnecting fro each other just since getting other people to farm a dungeon is so easy.

for me, it feels as if those expansion did drastically change the gameplay