How much better is it? Is it a huge upgrade or just marginal?
Other than the AG coating as blackened said, which was pretty garbage even for me (being someone that really likes AG coatings)...
1. Better colour support - that is, you don't have to deal with oversaturated colours of the U2711 (unless you ran sRGB emulation, which was only ok on the U2711).
2. You get a USB 3 hub instead of a USB 2 hub.
3. Much lower power use.
If you have a U2711 already, I doubt it's worth it to get a U2713hm unless you can sell your U2711 for the
same price as a new U2713hm. If you were looking to buy a new one, there's no contest in my mind - the U2713hm is the one to get. The U2711's bells and whistles are limited to colour gamut (for wide gamut photographers with a wide gamut workflow), SD card reader, and more inputs (+1 DL-DVI, component, composite). For the vast majority of people, the only real advantage the U2711 has is that it has an SD card reader.
If you're thinking of spending an extra $200 for a U2713hm... if you're relatively happy with what you have right now, then no, it's not worth it. Unless there are glaring problems with your enjoyment of your old monitor, don't bother.