Michael Cera pretty much plays the same character all the time, but I like that character, and it really met the needs of this script. The same is true of Jack Black. So, what we're seeing here is not an extensive character study at all, it's really just some guys fooling around in tunics and parodying the Biblical age. It won't get you an Oscar, but it makes for good entertainment.
That said, it's not like the script wasn't solid, or anything. It actually had a pretty legit story to follow and it followed through to its logical ending. A Judd Apatow film really has to, though, doesn't it? One thing about Judd is that his movie might have some irreverent and hilarious humor, but the story always has heart at its core. There's always some kind of emotional pay-off.
Year One was almost rated R until they went back and re-cut it. There's some sexual humor, some language, basic Apatow stuff (aside from a distinct lack of marijuana). I'm not sure where I was going with that except to just mention that it's on the high end of the PG-13 spectrum. The thing is, though, if you can't handle what this movie has to offer you, then you probably wouldn't get it anyway.
So go see Year One if you want to see some iconic faces of comedy lampoon the ancient age. That's all there is to it. You won't get a serious study of historic events, you won't get a deep character study, or a compelling analog to modern life. It's just a fun comedy, and you'll enjoy it.