When browsing https://stackexchange.com/sites you have to click on a tab in order to see what that particular site relates to. While in many cases it's obvious, in many others it isn't (eg. Stack Overflow, Super User and Arqade).
Ideally, you would be able to view this brief description of the purpose of the site when hovering over each tab with your mouse cursor.
I really like your site, but if I'm going to use it regularly, this is almost a deal-breaker: I, like so many people, value time-saving features above all else. I really think this will benefit all your users, including seasoned users, and will benefit you by increasing the popularity of your site.