On selector performance, here's an odd quirk - try creating a big page with a bunch of headings in it and then compare the performance of $(':header') vs the equivalent $('h1, h2, h3, h4, h5, h6') it seems :header misses out on a bunch of optimisations that are applied otherwise and using it turns out to be cripplingly slow.

A minor jQuery niggle - currently it adds an onunload event to the page which breaks fast back and forward navigation in browsers