1) Badges and Tokens: Drops are very hit-and-miss - the number of times we've sharded loot and banked the mats because noone can use it is horrendous. Adding token-based loot for T4 opens things up somewhat (since its not totally class based). Similarly, badges allow items to be earned over time even if loot drops are unlucky (or in the case of our DPS train, oversubscribed); I got virtually nothing of use in out first few months in Kara but quickly obtained some nice goodies from badge rewards once they were available; much better than spending 2-3 hours getting to a boss only to have stuff drop for a class that isn't present (Gandling in Scholomance anyone?) or a zillion runs for an item (14 runs for my class shoulders from Raz Frostwhisper, and an associate was up to 20-30 for one of his in Strat).
2) Heroic instances: TBC added in a lot of new instances, but by adding a second difficulty mode Bliz ensured that these'd still be relevant at level 70 (unlike so many old-world instances that languish unvisited).
3) Daily quests: A simple solution to the money shortfall, giving people chance to earn cash (and rep). The Rokk's cooking quests got my cooking to 375 and are still useful for supplying grub for my "family" and the guild bank.
4) Mac Video recording: Though I only occasionally play on the mac now I have a new PC, this is a great feature and I plan on using it more.
4) Epic Flying mounts: Zoom! What more need be said. Once tried, you'll never want to go back. The only drawback is that the world feels somewhat smaller (but just go back to a low-level alt for a reminder of just how big it actually is). For those that don't know I'm a mountaholic - I have 3 epic flying mounts (Griffen, Skyguard Ray and Netherdrake) and 5 epic land mounts (Horse, Stormpike ram, PVP Cat, brewfest Ram and Rep Talbuk) on a random autochanger, as well as a host of non-Epic versions :)
5) Guildees: Without the people I've played with for the last (almost) 3 years WOW woudln't be half the game it is. While there are clashes of personality and differing agendas, they're a great bunch. Then again, we have a specific rule that to join the guild you need to know at least one of the existing members in real life so its all "friends or friends of friends".
6) WowWiki: Stepping outside the game itself, without the institution that is WowWiki my knowledge of the game and its mechanics would be significantly reduced.
7) UI Addons: My hat is off to those who put some of these together, from the useful information display that is Titan and its plug-ins to the now-essential Omen (or KTM). One I find most useful is Outfitter as it allows me to swap kit about easily, automatically equipping speed trinkets and cloak when riding, swapping in +hit gear for boss fights (on a keysequence) or swapping in PVP gear in battlegrounds (automatically). Some of the logic is quite sophisticed - not equipping the riding gear in battlegrounds for example (as being shot of a horse puts you in combat and thus unable to switch gear better suited for fighting) or adding a time delay on unequipping riding kit (so that parachute cloak doesn't unequip as soon as you're shot off your mount, leaving you to plumet to the ground!)