What do you want to be able to customize?
anuraggoel: I am probably just rephrasing your question. It looks like a lot of the suggested customizations (except things like ranking algorithms) can be coded on a site-wide basis and enabled/disabled on a per-user-basis by just selecting drop-down values or entering text in a box, without users putting a single line of code in their profiles. Just like noprocrast, topcolor etc. Additionally, a user code fragment that provides useful customization may also be useful to others (like a greasemonkey script). So, what exactly do you see a code fragment doing that would be unique to a user?