Alright, I'll bite. Some background; I come from FFXI/FFXIV and have only played WoW for 3 days on a free trial. I played Aion for about 3 months. AoC for about a month.
1. Dev team - Trion has some of the most responsive developers I have seen in an MMO. The headstart was smooth, any bugs that were identified were fixed within a few hours, and there were no server crashes to contend with. When their queues filled up, they were quick to open new servers to funnel people into, which tells me they were prepared for this. There was daily downtime for 15-30 minutes, tops, which is better than having hours of downtime later to fix the same issues. They post in forums! Their Facebook page is updated regularly. All of this is worlds better than anything I've seen from Square-Enix.
2. Environment - I'm able to run on ultra, which makes things look great! The areas are varied, you can clearly tell when you're passing from, say, Silverwood to Gloamwood from the lighting and the scenery. Exploring is fun; while there's a few mountains that you can't climb and areas you hit an invisible wall on, for the most part, if you see it, you can get to it in some fashion
3. Zone invasions/rifts - I'll go ahead and say it; I love zone invasions and rifts. I've seen griping about frequency on the forums, but I can't tell if its a popular opinion or if they're just a vocal minority. In fact, I think they toned down the frequency within the last patch or two; I could be wrong. In any case, I like running around closing rifts when I come across them, I'm always participating in the zone invasions, its great fun for me.
1. Community - Maybe here, I mean mostly the official forums. Going to the official forums for information is a pointless task; the forum seems filled to the brim of angst and complaints about nerfbats and ragequitting and other such nonsense. If you say you like the game, you're an automatic fanboy (rather like how the FFXIV forums were, though on a much larger scale). In game, for the most part people seem helpful, though on my server there's a few notorious trolls that yank on people's chains in global chat for no reason that irritate me. I guess you get that wherever you go, but FFXI/FFXIV had no global chat so it seems to irritate me more than most.
2. Quick leveling - I'm 42 after a month of casual play. The cap is 50. Yes, there's content after 50, but my current schedule doesn't allow me the freedom to join many raid guilds. I'll probably end up rerolling a new class when I hit 50 and hope my schedule opens up sometime down the line to allow me to play my 50 some more. In the meantime, I wish getting to 50 was more of a task.
3. Auction House - A minor gripe only, the AH interface seems awkward. I can't put my finger on why, though. I just know it takes me forever to search/list things.