Stuff that I find exiting in cataclysm:
1: Being able to fly in the older world, let's be honest, walking sucks, and riding does too, I wanna fly and soar the open skies, and it has allways bugged me that in the old world I'm stuck at ground lvl, now on the upcoming cataclysm, I'll be able to fully explore every last corner of the world, no more hidden corners, no more annoying landscape making navigation dificult.
2: The Revamped talent system allows you to "feel your spec" at lvl 10 no less, previously you had to wait to at least lvl 30 to really feel your spec being diferent from any other, like, if you are a shaman, only at 40 do you truly feel like an enhancement shaman, being able to dualwield, use stormstrike, lavalash, ect. Now all specs of all classes can truly feel their specialization at 10.
3: Terrain Phasing, do I need to say more? as it is, the standard Phasing was pretty awesome in WOTLK, the way how the scarlet enclave passes from a paradisiac land, to a barren scorched ruin once u've done all the quests is quite unique and fantastic, same goes for the storylines in stormpeaks and icecrown that did made heavy use of phasing. Now on cata, we have terrain phasing further changing the way the world looks like as we quest, it makes us really feel like we're leaving a footprint on the world as we live our adventures in it.
4: Actual strategy being needed in instances, in WOTLK most heroics (with the exeption of the last 3) were easy to sum up, with: Tank groups them up, party AOE's them down, rinse&repeat. Not anymore in cataclysm, with caster mobs having to be interrupted, healers having to be CCd or silenced, single-target nuking being a must-have, a target-priority system for the special encounters, healer actually having the inherent risk of going OOM if they aren't carefull, and more. It will make instancing feel exiting again, sure this will make the farming of Heroic poins quite a lot harder, but also more dinamic instead of the boring experience it is now (I can literally read a book while I tank most heroics, I finished reading "Don Quixote" while farming my pre-raiding tank gear :P)
and the best for last:
5: a complete and (god knows much needed) revamp of all the old questlines. Don't get me wrong, I love the old world, it was beutifull, and some quests (like Tirion's redemption, or the battle for darrowshire) were quite epic, but being completely honest, quests from 1 to 58 feel horrible, the pacing is nasty, the quest-desing is boring, the quest rewards quite pathetic (and from the aesthetic POV quite ugly too)and it makes the act of leveling an alt, feel like a tedious chore, but this will not be the case anymore, now some of the best quests are from 1 to 58 (Can you say with me STONETALON MOUNTAINS) wich will give this game something it lacked previously, HUGE REPLAY VALUE, the act of starting again will feel fun and exiting once more.
there's other features I like (like the new guild system or reforging) but these pale in comparison to these 5.