Please don't take that as an attack TemporalFlux, just curious about why you felt that way.
It really is just the way it feels for me. I don't think I could give sufficient quantifiable evidence to win an argument; I'm hard pressed to play every game on each system.
For me, I think it bridges back to the original XBox compared to the PS2. We had the usual PG-13 fare going on with PS2, and suddenly we had Halo with characters casually screaming "Oh shit!!!" every minute or so. For me, that was probably the defining break between the two systems; and I haven't seen anything since to change my opinion.
Certainly there are games that cross systems (like Grand Theft Auto), but those games aren't really a signature of a system. For Nintendo, the signature is Mario; for XBox, the signature is Halo; for PS, the signature is less defined, but the best candidate is probably Metal Gear; and Metal Gear is a teen franchise in my eyes (somewhere between Mario and Halo).
In the end, it's all just opinion; I'm not claiming I'm right or that someone else is wrong. But I think we can agree it's a bad fit for XBox to drift toward something cutesey that Nintendo popularized. XBox shouldn't be cutesey; it goes against its brand.