My top reasons why WoW is now the best it's ever been:
1. Leveling is possible without monotony. Getting xp from everything from BGs to gathering, as well as instances and the traditional quest-grind, makes the game perpetually fun.
2. Pretty much all classes are just so much better than they used to be - the new specialization from L10 makes all specs viable from L10, and the improved stats system means that agi-dependent classes do not have a huge advantage over SP-dependent classes until L60 like before.
3. BGs from x0-x4 seperate from x5-x9. This is game-changing for me! I used to dread hitting x0 because it meant many levels of grinding before I could enjoy BGs again. Now it's seemless! This one just might be my favorite change overall.
4. The new content is just SO much better than the old content. I know - we all knew it so well, and loved some of those quests, but the game is just so much better now. The quests flow much better, the objectives are more varied, there is less time spent wandering and more time spent doing *fun* things.
5. Rated BGs - Okay, I haven't done this yet, but the prospect has me tremendously excited. I have always had a love-hate relationship with Arena. Firstly, it is based almost completely on twitch reflexes, rather than thought, and my reflexes are definitely not as fast as some people. I struggled to get past 2000 (lifetime high was 2040 in S1 5v5) because the game was just too WHAM-BAM-DEAD for my liking. Way back in Vanilla WoW, I co-lead a guild that ran BGs and won most of them using superior strategy to trump better gear. It was a blast. There is so much potential for strategy in games like WSG and AB that has gone untapped - until now. I'm FAR from ready for BGs ( my new toon is only L27 right now) but I am so excited for when I get there!
6. Goblins! Thieving, playable Goblins!
7. FLight paths and graveyards everywhere - running across huge zones is a thing of the past!
8. Guilds - Finally have meaning! The rewards are awesome, and the motivation to actually do things as a guild seem like they could really help bond a good group of players together.
I could keep going and going ... I just love what they've done to this game. I play "casually" now, and I feel like any hour I want to spend on WoW I can always find something fun and productive to do.
Another 4 hours and I'll be home playing again instead of just talking about it ;)