you miss out on basically nothing. There is some added bit-depth capability which could be taken advantage of more in the future, which is nice. But you can already do 10 and 12-bit with current HDMI if you're sending YCbCr. IF you're using RGB, then you are stuck with the 8-bit bottleneck (same as DVI) which can sometimes be a problem if you're doing processing and such in the source or earlier in the chain.
Please, educate me here. So if I get a PS3 which will have support for HDMI 1.3, but my TV set only supports HDMI 1.1 and so does my AV Receiver, could I still connect the PS3 (HDMI 1.3) directly to my AV Receiver without seeing any problems whatsoever? In other words, does HDMI 1.3 have backward compatibility with 1.1?