1. Class Design
There are very few skills that you never actually use and very few skills that replace old ones. Compared to many other games, both single- and multiplayer, it's great that you don't just throw away half of the skills you get. The pacing was fairly good as well - it gets more complex as you go. Now that I've become accustomed to the complexity and pace of 60, it naturally feels rather boring to be synced to 17 and lose all my skills, but when I first started out, I actually found it rather good, because it gave me ample time to learn.

2. Visual Design
A few female-only attires aside, I'm quite happy with the way things look - not just for armors, but player and monster models and dungeons as well. Not everything suits my taste, naturally, but seeing as diversity is to blame for that, I can't find a fault in it. I'd just wish for more racial diversity, since everything is so human-ish >.>

3. The Lore
I have to say beforehand: I came from WoW. And the way they mishandled lore was...appalling. Frankly, if you want to offer Worgen Deathknights, don't write into your lore that Worgen are immune to undeath, choose between either. I wasn't fond of the Sargeras retcon that created spacegoats either, especially since the excuse was:"Sorry, I forgot and was too lazy to do research on my own works.". Don't even get me started on green jesus - you can't expect me to take your lore more serious than you are. So I am more than glad that thus far, the amount of retcons and horrible writing is kept well within my tolerance threshold.

4. The community
There are a few arseholes out there of course and the forums in particular often make me angry to the point of struggling for self-control, but ingame, the vast majority of people I met were really nice and helpful!

5. Relative Lack of RNG
Really, it's a selling point to me. I absolutely hate having to grind random drops.

6. Clarity
Almost all you need to know about your skills is spelled out in the tooltips. Nowadays, almost every mechanic has its own indicator and whenever that mechanic pops up, the same indicator is used, making for consistency. Moreover, many indicators like the stack marker make it very obvious what you are supposed to do. That eases the learning burden tremendously.