watts rms, xmax, resonant frequency,and the magnet wieght, though i wish it told what the magnet is made of or a specail rating that tells how strong it is. i also look at the voice coil length and diamiter. and the cone type because i read that a paper cone gives better sq.
well the main thing is brand name for SQ. Try and listen to as many subs as possible or go to shows and see what these subs sound like. sometimes you do have to go by word of mouth if certain subs are rare and not sold in most stores like RE. It's all about experience w/ all of these popular subs and hearing them a bunch in all kinds of setups.