World of Warcraft: The Burning Crusade, the first expansion and the most loved one
The Burning Crusade has been the "best" expansion so far according to most players. I'm using quotations since its relative to each and every one of us. This expansion brought a new realm to the WoW universe, since Outland, this expansion leveling/raiding zone, is not on Azeroth.
Besides Outland (which includes new quests/zones/monsters), this expansion raised the level cap to 70, professions got a new level (Master), flying mounts were introduced, and they also added a new race for both the Horde and the Alliance. The Horde side got the Blood Elves, and the Alliance, the Draenei, both races of course got their unique racial traits and specializations
PvE-wise there were some changes, there are no more 40-man raids this time. They are all either 25 or 10 man raids, and the classic 5-man dungeons now got a new mode intended for max levels (70), heroic modes. These heroic instances are the same ones you can do while leveling up, the difference is that they are tuned up for 70s instead of lower levels, and of course, the loot is for maximum levels as well.
World of Warcraft: The Burning Crusade Cinematic Trailer
PvP-wise they added new battlegrounds, a new pvp stat called resilience, and the Arena. What that PvP stat does is that it reduces the damage you take from other players, making it convenient for players to get PvP gear for PvP instead of just doing everything in PvE gear. This right here created quite a debacle, since raiders argued that it was way easier to get PvP gear that was equivalent to those of raids, and if you tried to PvP with PvE gear, you'd have a hard time killing other players due to resilience.
Finally, the Arena. I think this was the best PvP addition given that is A LOT more fair than battlegrounds. Here, you'll just have whatever buffs you and your partner(s) can give to each other, and some spells are restricted, given that it'd be too much of a disadvantage for your adversaries. Plus, the rewards for being in the top 0.5% were awesome, a really cool (and fast) mount and a Gladiator title. So for all those players out there that like bragging about their accomplishments, this is the place to show what you've got.
Considering all the pros vs cons there are in this game, I think Blizzard did quite a good job on continuing this already exciting and succesful game.