I wasn't terribly thrilled with Death Knights when I first saw them. I didn't like the idea of yet another plate class (since I play a lot of warriors), and their whole "recently freed from the thrall of the Lich King" deal didn't do all that much for me. They didn't seem that different from Paladins, to be frank (which is because they're not: thematically, they're the old D&D Anti-Paladin/Blackguard idea) and although I dabbled with them from time to time, I didn't think I'd go the distance.
The first crack in that sense of disdain for the class was the starting zone experience. Anyone who's done it knows it's one heck of a well designed start to finish piece of gameplay. It was so well done, in fact, that I actually rerolled several DK's in an attempt to find the one race I really liked, secure in the knowledge that not only would I start at level 55 and be Outland ready by the end, but I'd enjoy the trip. After that, I blew through Outland on a Tauren, Gnome, Draenei and Dwarf before finally settling on my Night Elf here. Heirloom shoulders made the already painless starting zone even more rapid and Outland fell away like a solid fuel booster rocket lifting me to Northrend before I'd even finished Nagrand.
Now here I am at level 80, and for once I actually have a third alt at max level (not counting the old days when I had four level 60/level 70 warriors - I'm sad to report I only have three 80 warriors now) of a different class. Gearing the DK's been interesting, and as I've run myself through various content my DPS has been steadily increasing, my understanding of my specs steadily expanding (frankly, I love Blood as a DPS spec, it has so many cooldowns for AoE damage and a nice bag of tricks without being as fiddly as Unholy, and I can't say enough good things about Frost as a tank spec) - I have to admit, I've totally turned around on DKs. They're awfully broken in my eyes (even after the most recent nerfs to their cooldowns and tanking health) but I cannot pretend to be having a bad time playing the class.
So now I turn to you. What class turned out to be a surprise hit with you once you tried it out?