I voted 25 points though I prefer 28-32 points.
I've built characters on 22 points (a cohort for Living Greyhawk) but I don't like the way lower point buys (like 22 points or less) force characters to really really dump "non-essential" stats to be effective. The ranger/rogue cohort I made had 12 str, 15 dex, 14 con, 10 int, 10 wis, 8 cha which is enough to be a marginally effective melee rogue. Drop those stats any lower, though and in order to be an effective combatant, she needs to have an 8 int and an 8 wis as well. (Her strength can't go down because she needs to be able to carry a chain shirt, rapier, and a couple other things and still be at light load; dex can't go down because she needs AC and to be able to hit things, and con can't really go down because rogues need all the hit points they can get). I don't like my characters to be so one-dimensional.
Also, I suspect that the extremely low point buys would screw up some of the game's balance. At low point buys, stat increases like rage are more significant. Also, spellcasters have a lot fewer effective options--if you've an 11 int for your wizard, you'll never get 2nd level spells and your DCs are 11--13 if you have greater spell focus. Since that means your foes will usually have a 50% chance of saving on their weakest save, if you use spells with saves, you have a 50% chance that your 2/day ability that's supposed to make you special does nothing. At that point, spells without saving throws like magic missile or true strike look even more appealing than they do in normal D&D.