I am a huge fan! I am enjoying Vengeance immensely, but I agree it's not as good as the two seasons prior.
The casting changes have not benefited the show. The new spartacus is competent, but lacks the gravitas, the soul, of the original gladiator. Spartacus' newfound youth is quite jarring as well.
The original Naevia was stunningly hot. The new girl, not so much.
Crixus was always a beast in heat in the former seasons. Now, he looks old, tired, and depressed. Freedom has not been kind to him, lol.
The biggest disappointment so far has been the glaber character. In prior seasons, he always had a gleam in his eye after doing over spartacus, in a relatively small role. He smirked, and then he left.
Now, he's essentially the co-lead, and he just seems wooden, inexpressive and just plain dull. The political maneuvering within the senate angle just seems like so many trivial details. Craig Parker just cannot seem to emote. He almost looks like a robot.
John Hannah and Lucy Law were and remain so, sheer perfection. If I were to pick the best actors in all of tv, those two would be at the top of my list.