1. Leveling Wrath of the Lich King was nice only in the starting zones probably, Zul Drak or Dragonblight didn’t had anything epic, just some experience grind. The final leveling quests in Icecrown were the Blizzard test for the new type of leveling, instanced and linear. Vashj’ir and its worst alternative, Mount Hyjal changed the leveling experience. Questing became linear, having some sort of start and end. The level design of Vashj’ir and Uldum were epic and enjoyable at least for an MMORPG.
2. Getting rid of the minor issues. Bringing guild perks allowing fast movement, teleport, resurrection for any class is a great thing. Removing the requirement for hunters to level their pets, removing arrows, pet happiness. Removing the disadvantages for every class and making them really enjoyable.
3. PVP Non Arena balance. Wrath of the Lich King was worse mostly because of the survivability. Healers could survive against 4-5 players, they could beat every class and spec in duels and 1vs1 encounters, overhealing the damage and having some damage tools as well. Skill became more important than the class, every class can now die, even ferals that are very good 1v1. Probably melee classes cannot beat mages but skilled players will always win.
4. PVP gear with no rating restriction. No more non resilience players in battlegrounds or at least very very few because now everyone can get pvp gear with no restriction by just farming honor or do some casual arena. Some classes have a very huge disadvantage not having the best gear or weapon. Cataclysm fixed this even tough the concept was implemented near the end of the previous expansion when the Wrathful gear became free for honor.
5. PVE Cataclysm was harder than in Wrath of the Lich King. Blizzard listened to the feedback and made Cataclysm much harder trying to add some oldschool concepts. Destroying 25 man raiding was a big mistake tough. Not only the raids got crippled but also the guilds. The idea was good, the implementation was awful.
6. Is getting over fast. Already there are rumors for the next expansion that will probably be announced next month at Blizzcon. Ghostcrawler already said something about one or two expansions in an interview. Blizzard will take notes of the mistakes they did in Cataclysm and make a huge overhaul in the next expansion if they plan to keep the game alive. Nothing can replace World of Warcraft because of the fantasy world, the technology didn’t advanced so fast so a new graphic engine and character models is not really an option and also that Blizzard can create a new MMO game that will obviously not or cannot target the WoW audience.
7. The game is still the same. Players are doing the same thing they did in Wrath of the Lich King with minor changes. Who wants to have the same fun can have it. Blizzard managed to keep the game at high standards and other MMOs failed or at least they don’t have the same success as WoW has and they will never have.