> I'd say the second biggest reason is the annoying user interface
DuckDuckGo is the first website that made me want to install a browser plugin to change the way the page looks. (As for other websites that had a look I didn't like, I simply stopped visiting them.)
The problem with installing plugins, though, is that I use several browsers on several computers, and to get a consistent look, I'd have to customize all of them (if possible).