Not sure what you were expecting out of a story based on history. It'd be like making a WW II movie, but letting Hitler win.
I think they gave him about the best ending they could really. Ending it with him still drawing breath, ala King Arthur/Avalon style might have been slightly better, I suppose, which is what I had always thought the show would end with, but it was close.
Overall, it wasn't the strongest season. Spartacus, the show, was always best in the arena (Season 1, and the interlude that centered around Gannicus), and I know the producer?/writer?/director? has mentioned how in retrospect they should have spent a few seasons in the ludus, prior to breaking out, but they hadn't expected to do as well as they had, so wanted to get the story out "fast".
I enjoyed seeing a different take on Caesar (as well as Rome as the enemy), and I can only imagine there are writers already feverishly attempting to write screenplays to sell to Hollywood over a young Caesar story that differs greatly from the more traditional, cultured statesman that the plethora of Roman media over the years has given us.
Crassus, as the antagonist, didn't particularly live up to his promise of the first episode, and nothing really can stand up to Batiatus and Lucretia who were fantastic in Blood and Sand/Gods of the Arena. I guess it was a nice touch for Crassus, as a character, to show he's not entirely caught up in all the rhetoric he spews, with his little heart to heart(s) with Spartacus in the finale (though I'm also glad Spartacus didn't reciprocate such feelings, as that would have been a little too mushy
).
Still, while any more seasons would have overstayed their welcome, it's still sad to see Spartacus end its run. At least Starz has a new show which looks promising if the first episode is anything to go by, Da Vinci's Demons. Not quite as Chuck Norris as Spartacus, but still looks like it'll be a fun ride.