In the defense of WoW (on these forums, lol), I wanna say a lot of what used to be group quests got changed to not be group quests anymore around the time Cata came out. I could be wrong, though.
Regardless, though, it was possible to quest solo to cap in WoW. There used to be a few points where you'd pretty much have to stop and grind out a level or so, but those vanished during WotLK. . .