Given the bazillions of teeny tiny web stats packages out there, this wouldn’t normally be worth a mention, but it has a number of features which are pretty cool and not often found at this level:
Interface - the Mint interface is very, very simple (there aren't even any charts, at least in the version I've seen), but does have a nice feature that lays out multiple tables on the screen intelligently even as the screen is resized. This is paired with a jump-to navigator in the top bar of the interface, which makes it easy to get to a particular table of data (and the jump is nicely animated, too - or that may just be IE7).
API – the most interesting thing about Mint is its API, Pepper, which allows people to write plug-ins for the app which display specific kinds of data, such as outbound clicks on Google AdSense ads (something Google Analytics doesn't do). Pepper has become pretty popular – see Peppermint Tea for a list of current plug-ins - and is a stroke of genius on Shaun's part, since the effective functionality of Mint is now far greater than anything he could have created on his own.
RSS – information can be made available as an RSS feed. This is fairly obvious, since RSS is becoming the new e-mail, and web stats packages have been able to e-mail reports out for some time now (and many of them make their data available in XML format). But it's a nice little touch, and given than Mint is aimed squarely at the self-hosting/blogging community (even though you can't run it on a shared blogging service like TypePad), it's very much up with the zeitgeist.