Offered more content, but in doing so, made previous content meaningless, which made the game feel smaller.
Unfortunately, it made all previous raid content, and most team based content, completely useless.
Granted, that's progression, but alot of the old content played a significant role in both the lore and progression of this game for many players.
It still exists. I know some people that used to go back to it, from time to time. Hell, I even 3 manned Ony once, just to see if it was possible (and it was) But, outside of getting some personal kicks, all that old content is now dead to players.
Not that the old content is worth doing anymore, which is why it's not done anymore. It's that Ony, Neferian, Ragnaros, and C'Thun are significant characters in the WoW universe, and now the lore of that universe is skipped completely.
Also, forcing what were large raiding guilds to funnel into a 10 man instance as their frist souce of raid content was unkind. It tore apart alot of guilds in the game, and while mine survived, we definitely felt the splintering and defections as we tried to squeeze 50 active raiders into multiple groups of 10.
Outside arena (which the significance of is debatable), PvP has seen no major upgrade in this expansion, outside of loot. Eye of the Storm, while fun, was uninspired. And class balance has actually become alot worse, and surprisingly, seems to get worse with every fix. Good players adjust and play through it, sure, but it's obvious and only ignored or played down because of the many times it has been pointed out, which doesn't make the actual point of imbalance invalid. It just makes it common.
Griping aside, the content itself was well done. The territories are wonderful, and the amount of dungeons added were abundant. I doubt a single person could argue that Burning Crusade offered a world with tons of things to see and do.
I just wish there was a way to add all this content, without making the content I spent 2 years playing in, feel completely useless.
In that way, this expansion actually made WoW a smaller world to play in, and made all the time we spent in Azeroth, almost completely meaningless.