1. I havn't bought a game based on phrases or words (on the box) since I was very young. But back then I think I'd be swayed by an exciting cover, and if the back told me I could play "over 10 levels!" watch "many hours of CG!", anything like that. Yes, I was easily swayed!
2. Reviews play a part, but it's primarily what my friends think (by 'friends' I mean people on games' forums, how sad is that?). Another factor is if this is a sequel to a series I like, that way I'm immeadiately interested.
3. Personally I'd feel distasteful using humour for a game depicting a world war, but from a marketing perspective, if you threw some humour in there then people are bound to notice it, or pick up on it. So I probably would.