Have to admit I find the issue of finding the Surface Pro too heavy interesting, it's a non-issue for me. I go from using an iPad at my girlfriend's house, to using my Surface Pro at home and obviously there is a difference, but I really don't 'feel' like there is one.
It speaks volumes that I'm sitting here trying to think of something that's really disappointed me. Off the top of my head I'd say that the touch gestures and on-screen keyboard are not quite as good as Apple's... but then again this is the first version, I'm sure it'll improve. Maybe I can think or something better (or rather, worse!) later.
It's really mostly when I'm sketching, I just wish it was a bit lighter. In truth I just pull the keyboard off now to do that and it's fine but I'd rather not have to.