I hadn’t run the beta builds of Mavericks, so I was a bit affronted when I installed it last night to find that it would change my wallpaper without my will. Aside from that, there is little to complain about.
A lot of the new stuff isn’t user-facing. Everything has been retouched, nothing (except for the power efficiency changes, perhaps) is dramatically innovative. This doesn’t bother me, but I can imagine people would have irrationally felt ripped off if Apple had charged for it.
For instance, Mavericks uses a slightly tweaked version of Lucida Grande for the system font. The difference is minor but it is noticeable to me on a Retina MacBook Pro. The best way to describe the change is that the strokes feel slightly crisper.