I find Cataclysm is a hard expansion to really pin down. The thing is that there were no "unambiguous wins" this expansion. It seemed like every element was "one step forward, one step back".
Compare that to Wrath of the Lich King. Lich King had, in my mind, at least two moments of awesome: the Wrathgate ("Did you think we had forgotten? Did you think we had forgiven? Behold, now, the terrible vengeance of the Forsaken!" - best speech since original Ragnaros); and Ulduar. Maybe the Lich King fight counts as a third moment, I'm not sure about that. Cataclysm, in contrast, didn't really have anything to match those highs.
- Choosing specializations at level 10 - I thought this worked remarkably well. It made choosing your path a lot cleaner and simpler.
- Class balance in general - I really liked class balance this expansion. While there were a few issues at the very high end, for the most part almost all specializations were balanced against each other.
- The new healing model - Maybe it's because Holy paladins were one-button spam for so long, but I greatly enjoyed healing this expansion. Using many different spells, watching your mana, triage. The basic process of healing was interesting again. It did break down a bit near the end, admittedly.
- Twilight Highlands - Dwarven wedding! The best zone of the expansion.
- Mylune - You know you love her.
- Looking For Raid - excellent job on making a transient version of raiding.
- 10-man raiding - Pro: 10-man raiding became a first-class activity. Con: 25-man raiding was gutted.
- Guild levels - Pros: was fun to level up your guild. Cons: loss of perks made guilds too sticky, I thought. Also, the setup of cauldrons and feasts was terribly annoying, and done just to preserve the perk.
- T11 and Firelands - Pro: solid, interesting fights. Con: very static difficulty, gave rise to "dancing" claims, also possibly too difficult at the start
- Firelands dailies - Pro: Interesting quests, liked the storyline. Con: because of phasing worries, actual process was excessively grindy. Unlocking areas every single day was annoying. Blizz should have just had it unlock once and not worry about the phasing.
- Heroics - Pro: difficult and challenging. Con: difficult and challenging. To be honest, I still think these could have worked if the endgame had been structured differently. Take a look at how TBC heroics were placed in the attunements for raids
- Hyjal - Pro: good quests, interesting storyline. Con: excessively linear.
- Deepholm - Decent zone, nothing too interesting though.
- Uldum - Pro: quests with the cat people: Con: quests with Harrison Jones
- Old World Revamp - Pro: Lots of interesting new quests. Con: Loss of the old stories and common experiences. Many veterans felt it was too easy.
- Dragon Soul - I don't think I can look at this raid objectively. My feelings are coloured by the dissolution of my guild.
- Archaeology - Pro: neat items and very flavorful. Con: Very grindy. Fly a lot, survey, survey, survey, fly to the next area.
- Vash'jir - Underwater sucked. Was way too long and linear. Trapped a quartermaster midway through.
- Only 5 levels - Questing was too short, endgame came too fast. I much preferred the 10 levels of the previous expansion.
- Deathwing - Pretty boring villain. No style. Didn't appear often enough to feel like a true Big Bad. It felt like Blizzard thought they overexposed Arthas, and tried to pull back with Deathwing. But the problem wasn't that Arthas appeared too much, it was that Arthas always lost, and so started to come off as a paper tiger.