It's right on their site:
+ More of your "favorite" shows, longer episode retention
+ More devices
+ More pixels
The thing to remember is that it's not whether you're a plus member or not that determines if a show is available, it's the device that the content owners agreed to. Of course, only plus folks get the TV/Mobile option at all. You don't lose anything with Hulu+, you can still watch everything that you watched on the PC, on the PC. It's just that you can't watch everything on the TV that you can watch on the PC. I suspect this will improve over time as old agreements expire and new ones are negotiated.