Actually, Rome was originally going to be a mini-series, but was expanded to a full series based on a 2 year commitment. There were concepts for 3rd - 5th seasons, but each season was conceived as its own complete story, so there are no cliffhangers. It's probably the most lavish and expensive series ever done, and that's part of the reason that more seasons didn't get made. Instead, seasons 3 & 4 were telescoped into season 2 to wrap up that story completely.
In its way, Rome is at the same level as Deadwood.