As has been brought up, you need the hand seals to do jutsu (Otherwise orochimaru would still be able to do jutsu easily after losing his arms wouldnt he?). Choosing not to animate them is a descision to avoid boring repition of them.

Oh yeah, theres no one handed jutsu per se. Only Haku has that ability and he's dead. Everyone else (so far) has to use both hands for jutsu.