I upgraded from CS3 to CS6, and the speed alone made it worthwhile. I like the autosave and mini bridge, being able to change the label colors of multiple layers at once (something I use when adapting templates), and the darker interface (though I hated it for the first week or so). I'm sure there are some other new things I use, but they're not that important to me. I'm sure that as a designer you'd use more of the features that I've heard about but not really bothered with.
It's been a few months now so I could be remembering wrong, but I didn't have much of a learning curve. A few things had changed locations, but the features I use the most were still the same. The one thing I've struggled with a little has been sharpening filters. The ones in CS6 seem to be stronger than in CS3, so my purchased sharpening actions don't work as well and I haven't been able to replicate them well. I haven't invested time in reading tutorials or anything, but the little tinkering I've done has only been moderately successful.
As for a list of features, there was a good list (or was it a video?) linked in the thread about CS6 that we had going earlier this year, when people were trying the beta version.
