We traded in our 2011 RTS for a 2015 RTS last October. The motor, as you've seen, is the biggest improvement overall - vastly improved mileage, lower rpms, etc. Most of the other posters have listed the other items, although collectively they are not as big as the various things the 1330 provides.
There are a few other things to consider as part of an upgrade consideration that may or may not be of consequence:
The 2015 comes with floorboards instead of pegs for the rider. At first I thought I would not like that, I rather liked the pegs; but I have changed my mind and actually like the floorboards. I still added some highway pegs for stretching out on the road.
The seat has a hole, covered by a plastic seal, to add a backrest. This makes installing a backrest far easier than cutting the old seat. After considering the options, I went with a Utopia like I had on the 2011, very happy with it.
The front is redesigned as a result of the new motor and relocating the radiators (the latter helps the heat issue considerably). To my eye it is not quite as pleasing a shape, though not offensive. The biggest drawback there is the frunk is smaller. We used to be able to fit both helmets in there, now only one will fit because the front to back depth is a bit less.
The front fenders have been redesigned (which was done on the 2013 actually). We had the LED strip lights in the crease on the front fenders of the 2011 but that is not an option on the newer bikes.
The wheels (unless you get the chrome ones) are black and a different design. Personally I preferred the 2011 wheels but just a matter of taste.
On the motor, one very obvious difference when you start it up is that it sounds nothing like the older twin. Personally I don't think it sounds very "motorcycley" anymore (in fact it sorta sounds like a John Deere to me). Certainly easier on the ears for long rides. But it was actually one of the few things I really didn't like about the new ones. I decided to buy Lamonster's Performance Exhaust pipe and am having that installed next week - the sound (based on his video clip) seems better.
But as was noted by Chupaca, if you are on the "RT" path, this is a singular improvement in nearly every area aside from those I noted above. I am very happy with it, and looking forward to having this one a long time!