The big four. Dojo, MochiKit, Prototype/Scriptaculous and the Yahoo! UI Library are the top of the pile as far as I’m concerned. They cover the bases effectively and each one offers something interesting that makes it worth studying in its own right. If you plan to evaluate some existing libraries these make an excellent starting point.
Leaky abstractions. Joel Spolsky’s essay The Law of Leaky Abstractions is my favourite of all of his online articles. It’s basic tenet is that abstractions that are designed to save the programmer time inevitably leak, and if they leak at a lower level of abstraction than the programmer is familiar with they prove almost impossible to debug. Paradoxically, the more time saving abstractions you are using the more you actually have to know.
Community and documentation matter. As with all open-source software, it’s a good idea to get a feel for the amount of community activity around a project before you commit to building on it. The big four all have active communities, which means less bugs, more support and a faster rate of improvement. Likewise, good documentation is invaluable.
For more accurate coverage of the @media panel itself, take a look at my co-worker Paul Hammond’s detailed notes.