I liked the show. I didn't think I would, after the first episode (those blood splatters looked like Microsoft Word ClipArt), but I warmed up to it when it broke what I felt were the norms for such a show.
Spartacus didn't immediately show up the sneering "SUPERIORLY TRAINED MINOR ENEMY" in Crixus. He got his ass kicked, as he should have. He didn't immediately win the support of all the gladiators (and still hasn't, entirely, from the look of things). And it didn't try to make you feel for the villains. In fact, if anything, it hit you with "villains" that you thought "Hey, they aren't so bad" at first... but by the end of the season I was thinking "Holy SHIT, he/she needs to die, and soon".
It had its share of surprising moments... or at least feel-good moments, if not surprising. Crixus' intervention in the bathhouse springs to mind, as well as the interactions between him and Spartacus just before the bout in the last episode.
In closing, I think the most shocking bit of character development was Ilythia. I loathed her at first because she was... well, stupid. Sort of a bubbly, thoughtless twat with no real redeeming qualities. Her evolution into the snake was quick in coming, but fairly powerful. I still hate her, but for more obvious, glaring reasons.
And I'm not ashamed to admit I gave a silent cheer when she got clocked by Glaber towards the end of the series.