Assuming cordless:
Foward/reverse is a rqmt. Don't laugh, they don't all have that!
Look for one w/ variable speeds. Some of the cheap stuff has a high/low but it sucks for driving in screws w/o variable speeds at the trigger. A good drill should go up pretty high in RPM's to allow drilling into hard materials (metal, concrete,etc).
Keyless Chuck is very handy. (For changing bits) The ones with an allen wrench (type thing) are a pita for swapping bits, but, are good for holding drill bits tight to prevent slipping.
A second battery is a nice option, you can have one charging while the other is in use.
I have an older makita, L shaped. It is very front heavy and I don't like it for that reason... Look for one w/ good balance from front to back (more of a 'T' Shape). If it's front heavy, you get tired holding it steady.
The volts stuff is mostly marketing nowadays, there is a difference, but, the higher voltage ones aren't necessarilly better. I'd compare that last, but, others may disagree.