I didn't get a chance to read everyone else's lists, so I'll probably find myself +1ing a lot of you.
Dincrest's showstoppers in no particular order:
1) An active-electronics bass that does not have an easy access flip-up battery compartment like Carvins do. I like things hassle free and though I prefer passive to active basses any day, I want my active basses to be as easy maintenance as possible.
UPDATE: 1.5) Active electronics on a bass costing less than $1000 retail. Most preamps on cheapies are garbage.
2) Excessive control knobs. It looks cluttery and sometimes may get in the way when I play the occasional pickstyle song. In addition, sometimes excessive tone shaping may seem versatile, but I prefer an instrument with one really awesome sound than one with a whole slew of mediocre ones.
3) A truss rod that's only accessible by un-bolting the neck, as in many vintage basses.
4) Pickups that aren't hum-cancelling. I do not like noisy electronics.
5) Vintage fingerboard radius. I'm not a fan of the rounded 7.25" or 9" radii. I like the nearly flat fingerboard radius that Warwicks have. 12" radius is great too. I like low action for slap and tap styles.
6) Weight/balance. If playability isn't super smooth and buttery, forget it. And though I don't want a 14 pound boat anchor, I don't want a 6 pound feather either. I'm very energetic on stage, I writhe around on the floor, I leap into the crowd... so I need a bass that reassuringly heavy so that it feels solid.
7) Poor workmanship. The obvious things like bad fret jobs, orange peel paint, bad neck joints, deadspots, and all the other things that scream cheapo quality. My bass is a workhorse bass and if I want to do the job right, the tool had better not be a cheap tool that will break on me.
8) I'm not a fan of Fender or MusicMan basses, so anything Fender or MusicMan I will avoid. I love G&L instruments, though. G&L stuff is awesome. I also will not buy anything Ibanez. I've owned 3 Ibbys over the years and while not bad basses, they're really not my bag.
9) Straplocks are a must. I don't understand why many high end basses like Manne use regular flimsy strap buttons rather than straplocks. A guitar or bass has to have straplocks.
10) Overly ostentatious (sp?) fingerboard inlays. No inlays is my preference (side dots only.) But small face dots don't bother me.
And the honorable mention showstopper for me is cost, obviously. My tastes tend to favor basses in the $1500- $2000 price range.
Ya know...I've held my peace in the face of some fairly galling opinions, but this is too much. A bass without a good barbecue sauce dispenser is like a bass without strings; friggin' useless! 
