Guidelines/best practices for CSS hover animations in web UI

What, if any, are the accepted guidelines and/or best practices for using CSS animated transitions on hover in a web-based user interface? When does it start to simply become annoying and distracting?

I've noticed it showing up on some technology related blogs (example - hover over links) and also on Stack Exchange sites in some places (such as the top navigation bar on security.stackexchange.com). It seems to me like it adds a nice touch to the interface. At the same time, I think there's probably a point at which it stops being a good thing and starts to be distracting. And some other high-profile sites, such as other SE sites (this one) and jQuery's site (example: the accordion UI demo) don't use it at all.