The most useful (and hardly advertised) feature is that it can operate in standalone and class compliant (CC) mode

This is a small yet significant feature that more interfaces should have

Do not neglect offline use when buying an interface, I've come to appreciate this feature after owning several devices which only work when connected to a computer and wished that Focusrite had released interfaces like these earlier

And what's more: it is not a feature available on every audio interface out there

It has a good feature set too for the price

Great build, solid, more features than I can use

One nice feature that's not so obvious from the literature is the ability to essentially chain Scarlett units together.

the other feature I really like is that you can't create different headphone mixes for each

dual headphone out/mix routing, a very nice feature at this price point

The mixer is as close to silent as you can get.