SATA OR SATA 2 (good for raid)
size (as you like..)
NCQ (supposed to be good...)
speed (7200rpm is good for general use, high performance HD's run at 10,000rpm, or if you move onto scsi (server stuff.. not worth it for regular computer folk) it gets even faster.
cache, 8mb is good, 16mb is great. 2mb sucks.