It would be nice to be able to sandbox user comments so that they could include more than just plain markup. For styles this is trivial: provide a [this-comment] selector, and cut out any selector that doesn’t start with it (would fail IFF a future version of css introduced a parent selector, but even then could be easily fixed). For scripts, I’d assume it’s quite a bit less trivial. It would be really nice for us web geeks, though.
I even wish I could offer a bounty on this. It would really be worth it, I think, if it were done correctly. Unfortunately I’m too young for most things financial—credit card, paypal, etc. Oh well. Hopefully someone will find it an interesting challenge.