I think the coloring and the font sizes they use are backwards (the light gray coloring, with the white and blue make it very hard to visually skim and see how many answers things have, for example) - the ability to identify topics / find topics is difficult to navigate. Keyboard shortcuts in the browser break. The nesting levels are too shallow to allow for deeper discussions - they limit it to mono-dimensional responses with one layer of comments on answers.
email addresses and links don't work in posting boxes.
And a number of other things.
The quality of the content is superb, however (although there is a bit too much grammar nazi-ism, which I feel actually detracts from the flow of information at times)
so, Yeah I like the site - but the UI makes it difficult for that use to be fluid and efficient.