1: T11 raiding. I hadn't had as much fun raiding since Ulduar. Twelve bosses, a full array of heroic modes, and eight months of constant challenges. In eight months of raiding T11, we never hit a brick wall boss. We never ran out of content to work on. We got progression kills on Heroic Double Dragons the night before the release of Firelands. Once they fixed a few of the obviously broken encounters, Magmaw and Double Dragons heroic 10 were beyond ridiculous in terms of their demands. Maloriak 10H's broken adds were offset by the equally broken lack of an enrage timer. However, once those encounters were corrected, T11 was a great raiding experience. Encounters ranged the full gambit of skill levels, and were never subject to the asinine blanket nerfs to current content that gutted the later tiers. The afterglow of T11 is one of the last things that keep me still raiding in Cataclysm.
2: Reforging. Myriad were the times in prior expansions where I would look at a piece of equipment and think If only I could get rid of this damn hit rating. Thanks to this fantastic tool, I can get rid of that damn hit rating, well, most of it, anyways. Balancing hit caps and expertise caps on my DPS characters, and the rolling CTC caps on my tanks was made immeasurably easier thanks to this feature, which also acts as a gold sink to limit inflation in the WoW economy.
3: Specializations. Prior to specializations, you would find some really counter intuitive moments in the leveling process that encouraged idiotic behavior at the level cap. Until roughly level 30, Retribution was the best tanking spec for paladins, because prot paladins were heinously mana starved until they got BoSanc, and had little to do outside of auto attacks and judgement. Retribution wasn't just viable for tanking, it was optimal. It was better at tanking than the actual tanking tree. It was even worse for druids who were incapable of tanking until they got thick hide to make up for the fact that they were otherwise as squishy as a rogue. Specializations changed all that. Prot paladins were the better than ret paladins at tanking right out of the gate, as they should be.
4: Profession Dailies. I hate cooking in WoW. I also hate fishing in WoW. First Aid I find pointless, but I do enjoy the howls of the tailors when they find out exactly how much cloth I've come across in my time tanking across Azeroth, and just how much of it was converted into bandages that subsequently got vendored. While the cooking dailies and fishing dailies in Dalaran were a decent source of income, they sometimes required me to go gallivanting across the countryside to find Rhinos to murder, or to fish of some obscure rock, millimeters away from the hazards of fatigue water. You turn it in and get a token, or a bag of vendor trash, and a pittance of gold. Wooo... All that changed in Cataclysm. Well, not all of it, but the bad parts became less bad, and the instead of meaningless baubles, they gave me what I craved, skill points. From the comfort of the Alliance's glorious fortress of Stormwind, I can level a character's cooking or fishing from zero to max, without having to spend any time away from the friendly confines. I can, in effect, level cooking without cooking. That way, when I hit level 85, and someone asks who can drop a fish feast, I don't have to discreetly maneuver my character into the darkest corner possible to hide from their judgmental glares.
5: Rated Battlegrounds. Arena is a joke. A couple people lock themselves in a room and wail on each other for some points. That's not to say that they don't have their place in the game, just as comedy has its place in the pantheon of entertainment. But if comedy was the only acceptable art form, and the Oscars were distributed based on who got the most laughs out of people, well, it's not the end of the world, but you can see it from there. Likewise, I found it laughable that the only form of PvP that really mattered for the better part of two expansions was arena, a dystopic world of flavor of the month comps and win trading. As a veteran, I've always found BGs to be more compelling pvp, because there's objectives beyond simply "murder the Horde". It prompts new strategies, more complex strategies, and a plethora of new roles. While the rating system is pretty well flawed, giving players an avenue to progress in PvP beyond the trumped up dueling that is arena, was a great move on Blizzard's part.