A) Sound. This is to some extent personal preference, in that I buy speakers that sound best with the kind of material to which I listen. I've found that as you move up the line, that's less of an issue, since there are fewer design compromises -- you don't have to choose between deep bass, say, and a sparkling treble.
B) Can I accommodate it -- if it won't fit or sound good in my space, well, it isn't going to be of much use.
C) Price -- can I afford it. Honestly, if you know what to buy, there's so much great stuff available for a song that this isn't an issue.
Anyway, over time, you come to associate certain types of speaker with a certain kind of sound. Since I first heard a friend's KLH-9's several years ago, I've known that I was a planar type of guy -- stats, planar dynamics, ribbons. They have a naturalism on acoustical music that all but the most esoteric dynamics lack. You also get to know certain brands, and which ones are known for reliability, sound quality, service, and bang for the buck.