I think Phil is right in saying the directors probably didnt want Hinds performance to dominate the show, there's so many other storylines going on like Octavian, brutus, pompey etc that the directors didnt want the viewers just craving for Caesar, they wanted the other stars to also shine, after all the main storyline in the first series followed the lives of Vorenus and Pullo, in my opinion they were the main stars of the show.
Back to Hinds Caesar, i thought his interpretation on the whole was pretty good, although like many of you say he was just lacking that bit of charisma, that air of confidence that set Caesar apart from your average roman, the X factor!