I keep playing Warcraft. It's stayed fun but still feels kind of grindy. The good change is there's multiple things to grind on: quests, random dungeons, battlegrounds. Get bored of one and do another.
One big drag is the levelling curve. Level 61 takes almost twice the XP as level 60, and level 71 takes almost twice the XP as level 70. The quest experience goes up accordingly; Burning Crusade quests give a lot more XP than classic, and Wrath gives more than BC. You can even game it, doing level 70 quests when you're 68 and flying through those two levels. Dungeon and battleground XP doesn't scale as much though. And ultimately you hit a big slowdown. Of course the journey is the reward, but it's hard not to feel a rush.
Economics are really weird in the new fast levelling game. With all the heirloom stuff my priest hit level 65 before he'd gotten the "looted 100g" achievement. Meantime, of course, he'd spent thousands of gold on training, professions, the occasional bit of gear. And there's a few big ticket items: 1000g for dual spec, 1000g for Northrend flying, 5000g for fast flying. It would have been very frustrating if I didn't have the gold and heirloom items lying around from previous characters.
Now I'm looking to level 80 and wondering if there's a way I can play the endgame casually. I wouldn't mind seeing the 3.2 and 3.3 dungeons before Cataclysm drops. I've never seen ICC or the newer 5 mans. But sadly the way to prep for those is to run a bunch of mindless random heroics first, I may get bored before I can get that done.
I started playing WoW because I had some frustrating free time on my hands. No interesting new games, was in limbo with the pilot training, and wanted something simple and mindless fun. But now I've got the pilot license, there's an interesting new game on order, and I've got some other projects warming up on the back burner. WoW works well as a time-filler, but I wouldn't want it for an avocation again.