I just had a user-interface epiphany. I'm using Mousetool, which clicks my mouse whenever I pause with the pointer, and that makes navigation that you have to hover over to know where it's going really annoying. This is an important consideration in the drive for accessibility. Then, on a technical point, a lot of the technologies that drive tool-tip-style popups are unavailable to various segments of the web viewing public. The "title" attribute on anchor tags (links) only works in IE. Javascript popups don't work on text-only and non-javascript-enabled browsers (and, again regarding accessibility, not on speech readers for the blind, either). And, I don't care what anyone says, the dynamic CSS positioning of elements on a page (like, the aforementioned tool-tip popup) does not work as advertised, in either of The Big Two.

A call to action for web designers: Remember that the best art is most beautiful when it fits its function perfectly.