You can already sort by any of the required/automatically-generated fields that show in the member list: name, join date, and number of posts. (Although the fact that the name is a single field rather than first/last name makes that less than useful.)
The real issue is that all of the other fields are strictly optional, so there will always be a (large?) percentage of members with missing data when you go to do a sort. Even if you allow for that (which any intelligently-designed sort algorithm can do), they're just raw text with zero control over the user's inputs...pretty much useless as meaningful sort keys.