What's original these days, anyway? Everything's been done, and Witch Hunter is no exception - it's unoriginal material at its best, though.
I genuinely enjoyed Witch Hunter. It has its laugh-out-loud moments, and I think the writing is excellent. It's reminiscent of the writing of Joss Whedon with its combination of drama, humor, and strong characters.
In fact, the characters might be the best thing about this series. Despite the fact that they're usually introduced humorously, the characters are always well-established and deal with drama with realistic gravity. We discover very quickly that they certainly aren't paragons of heroism. Many of them are selfish, but you don't dislike them for it. The characters aren't clear about their feelings or each other's; they lose their temper with each other and themselves - despite their strength, they become frustrated with their weakness in the face of crisis. I very much like these kind of developments. The plot itself may waver sometimes, but for the most part, it's solid thanks to the characters.
WH's greatest strength may be its greatest weakness, though. I personally don't see why you wouldn't like to see minor characters fleshed out. I certainly enjoyed the three brothers' story. But I guess that's the problem with WH - when audiences are motivated to pick up a series solely for its characters, audiences usually pick ONE character. And so when the plot moves away from that one character, you lose interest in the series until Tasha, I mean, that character comes back.