CSS - the antithesis of frameworks
There has been a little buzz about some CSS frameworks recently, such as Blueprint, although YUI's grids & other parts have been around for a while. I've kept an eye on these, but they have never seemed suited to CSS for one main reason.
CSS frameworks maintain the style sheet as a constant, and make you change the HTML.
This is in opposition to the aim of CSS, where you leave the HTML in it's most meaningful form, and use CSS to change the appearance:
Complementary to structured documents. Style sheets complement structured documents (e.g., HTML and XML applications), providing stylistic information for the marked-up text. It should be easy to change the style sheet with little or no impact on the markup.
I'm not holding that up as a holy reference that must be adhered to, and there are certainly situations where CSS frameworks are useful, but it just doesn't seem right to change the HTML to use them (for production sites).
Jeremy Keith mentioned their usefulness in prototyping, but equates it to creating cookie-cutter sites. Jon Christopher touched very close to this point talking about how a CSS framework passively removes a great majority of semantic value from the markup of a document .
JavaScript is a programming language that in some way suits being built up into a library, and the recent spate of good libraries can be layered on top of an existing site. However, the same concept just doesn't fit a mark up language like HTML, or style sheets.