1. Bad pickup location. Too many basses these days do not have a reasonable neck-position pickup option, and the pickup location there is key IMO.
2. More than 4 strings.

I know a lot of you guys won't consider a 4-string, but that's all I will consider. I just don't like the sound of a B string personally. A 4-string bass sounds perfect as is. YMMV
3. Preamps you can't bypass. I agree with most of you - I need at least a passive option on my bass, in case the battery runs out, or if I just want the passive sound.
4. Bad balance - any sense I get that neck dive is possible on a bass, it goes right back on the rack. I won't touch it. Too many bad experiences with a Thunderbird.
5. Maple fretboard - I don't really have an explanation for this, other than just personal preference for rosewood.
6. Corny finish - I don't really like colorful basses too much in general, but bright translucent blue or opaque beige paint finishes are really only good for a test drive. Won't consider owning one.
7. Corny body design - Now before you all jump on me, I just want to point out that this is only a personal preference thing...I don't like the look of 99% of Warwick basses, and all those little basses that look like them. It just doesn't suit me at all. I like a retro look to my basses.
8. Too many frets - Again, this is just a personal preference thing. I've had 24-fret basses before and I liked having that option, but honestly I almost never used the option. It's another one of those things that, to me, makes a bass look too "modern" and I prefer the vintage look. When I see a bass with 24 frets now, I get the same gut reaction I get when I see a kid riding his bike down the street with a helmet on: "ugh, what's wrong with people today?" I'm just an old fogey at heart.
9. Bartolini Pickups - Again, this is only just personal preference, but I avoid these like the plague. A bass with Barts just screams to me, "I am fancy! Look at how hip I am!" I'd honestly rather have Darkstars. I'm sure Barts are very nice pickups, but this also goes back to #1: basses equipped with Barts often have them right next to one another, way down toward the bridge, and I can't tolerate that.
10. Ugly logo, anywhere on the bass - one of the reasons I don't play Peaveys.
11. Bad headstock shape - one of the reasons I stopped playing Peaveys back in the day. They've improved considerably since then.
I guess I'm just kind of picky.