They only difference stat wise between them an half elves is half elves get +2 Gather Info and Diplomacy and have low-light vision, while Half-drow get rapier, short sword, and crossbow proficiency. A debatable trade, which helps some characters more than others (only realy helps mages, clerics, druids, and others with VERY limited weapon selection), compaired to the skill bonus which is always available. It's more of a flavor issue than a game stat issue.
Personaly, I'm not a huge fan of the Drow, I think they're overused, and could live with never hearing about them again. But I don't see anything wrong with the half-drow mechanicaly.
I like that they don't get it automatically. Why would half-elves get elven automatically? I don't see that they would if raised exclusively by humans, which could be the case. Both undercommon and the weapons in question are fairly minor bonuses, which could be offset somewhat, by being raised on the surface and having friends and a greater understanding of human culture.
Whatever. This proposal could use a section on half-drow personality and culture. Where they're found, how they are looked upon by others, etc. But a player could create this him/her self, I would think.
Well i wanted to leave a lot of the specifics open to player creativity. plus sence there is no real EnWorld map (at least none ive been able to find) i wanted to leave that for when there is some idea of a geography.