I always love the 30 range.. I seem to find a zillion quests to do. It's 40-50 that's a pain in the arse, because ZF and Maradon get boring, and I always seem to run out of quests. Hell, 50-60 seemed to go so much smoother than 40-50... But that's just me, I guess. (I'm at 48 on my druid now and am about to cry because I only have about one/two in each zone..)
Sometimes it takes more than just a little motivation to keep leveling. I wouldn't have gotten past thirty if I wasn't playing with people I liked, in a guild I liked, on a good server, and playing a class that I really really enjoyed. When I first got the game, I could hardly get past 20, because even if it was kind of fun, I just really didn't have much reason to play. That changed with time, but it did take a little effort.