Retrospective, arbitrary construction of funnels. If you need it and you know you need it, their implementation is jawdropping.
If you are tracking a single funnel which doesn't change much, that probably isn't that spiffy for you. For example, you configure Opens Trial Page, Trial Signup, Opens Plan Page, Signs Up For Plan as a funnel once and you're done.
However, if you're doing more sophisticated things, it can be a lifesaver to not have to tell the engineering team "Yo, we're reconceptualizing our funnels: you guys now have to do annoying modifications all over the site." Lets say you have a Facebook game with a Valentine's Day promotion and you want to see whether people who sent their bonnie lass a cow to show their affection are better or worse customers two months later. No problem, with a few clicks on the interface you can whip up a funnel that tracks purchasing the pink cow as a stage and then watch conversions after that. You don't have to tell engineering about it, since they already fired the "bought a pink cow" event. You don't have to wait a few weeks after the experiment to collect data (most funnel analysis software I've used requires a do-over any time you create a new funnel or alter an existing one except by concatenation) -- you have the data, you're just slicing it a new way.
The notion of preventing a turf war between the analytics side and the engineering side is really popular with some companies. They also typically have lots and lots of money to spend.