Source code that not only works but works well. It is important that not only does it work for the end user but if another coder had to look at it, he or she wouldn't even need comments to understand its structure and function. Neat features are often present in really beautiful code, but they're only there if they aren't crufty or otherwise kludged together. For example, I would assume that however the experience and reputation systems work on everything2, they must be driven by exceptionally beautiful code. Hacks can be present in really beautiful code, but, they have to be exceptional hacks.