1. Gameplay: Actually the gameplay turned me off at first but once I got.. shall we say addicted? to the ambiance of the game and exploring it, the gameplay grew on me. It has the usual auto targeting and fighting mostly stupid AI's, though Merlocs and certian others display some limited tactics, like netting you and then fighting from range.
2. Fighting+Animations: As far as the fighting experience, World of Warcraft is often touted as having the tightest, fastest, most fun fighting experience in MMO's because of its animations and so on. I'll admit they did a good job on most, though the night elf one are a bit too elongated (though still fast) and all are repetitive and look like you have some kind of back issues cuz you never bend in any new way (maybe I'm asking too much here for 2004).
Still, the fighting is fast if you are a rogue, though not so much if you're a resto druid unless you are in cat form, and even less fast if you are a prot warrior or a holy priest. Definitely very fun for the dps classes in the game, I loved my NE hunter and my undead rogue, and my troll mage etc... Shamans are very fun to play if you make them into a whirlwind of self healing death you can rule the lvl 20-30 bracket.
3. Battlegrounds: These were kind of an evolution of the Guild Wars arenas and were definitely a good thing for the genre, in my opinion. They can be too much of a good thing for your game, as evidenced by WAR of course, if everyone goes into the battleground and nobody shows up to play outside. But anyways, I loved Warsong in the lower level brackets, though I despised it at max level because it was all about having raiding gear and running an invincible druid with the flag at that point. I loved Arathi Basin the best, definitely very fun and lots of different points to contend, camp or distract others. Plus it had a time limit which was very much lacking in some of the other ones (Warsong and AV)
4. Exploring: Wow gave exp for exploring! This was new, and the way the map unfurled when you found new points of interest was fantastic. Plus when you found new things, they usually looked neat, and were worth checking out a little, you might find a new dungeon or quest or cave or something. Who knows.
5. 5 man dungeons: I loved almost all of the five man dungeons. They were challenging and fun but not impossible (except the last few on rare occasion with certain party makeups) Rarely did you have to even check to see if someone was a thief/moron/non-comp etc.. You just ran the dungeon. If they stole stuff you added them to your blacklist and went on with life having possibly wasted three hours, but its OK you still had fun and got rep and other stuff. Its all good.
6. 10 man dungeons: These required your guild or a more competent leader to rally the random friends and folks, but they were still good fun. Stealing was more disliked as it affected more people over more time, but it rarely happened since you usually hand picked people to run with you. I really enjoyed LBRS, UBRS, and my brothers really liked Karazhan as well went that came out. The only problem with 10 man dungeons in my opinion is that the game needs more of them and less 25 man junk.
7. Occasional World PVP skirmishes: I played on a pvp server and so we would have .. encounters with the scum sucking horde, or on my horde server with the self righteous alliance who are really a bunch of opportunistic creeps as well. These were usually fun and good natured chases through the brush and mob herds that resulted in one or several people ending up dead. I did enjoy it when we really outsmarted them again and again and again, with stealth or other means, and it was fun to predict that they would cry and call their fatty guildies and then trick them into doing stupid things, like jump off cliffs or follow you into bad places, or just charge five of them with only two on your side and take two or three down while they stand there stunned before they retaliate. (I did that with my warlock, though I admit I did 'fear' one of them.) Sadly world pvp in general against quest hubs has been pointless (but still fun) in WoW and WAR. I expect something different in Darkfall but perhaps I expect amiss.
8. Swimming! Why don't more games let you go underwater? WoW even has a couple of underwater sections in its dungeons, especially Black Fathom Deeps.
9. Flying! Not many other games let you see the scenery from way up high but WoW does just that. This is nice and provides relaxing down time views. At the same time, I'd like to be able to get off sometimes, or just swift travel, but oh well, you get what they make for you. Enough said.
10. Ambiance: Maybe I should have made this number one. WoW has ambiance. Creepy places feel creepy, Night Elf places feel naturally beautiful and magical, Gnommish places feel ridiculously complicated and mechanical, Dwarven places feel solid, orc places feel starkly barbarian human places feel medieval but still full of adventure around every corner, The battle cries are great, the emotes are funny and sometimes crude, the characterization is unparalleled, and there are very few warps in the game other than hearthstones and warlock ports so 'immersion' seldom breaks.