I transitioned both our Windows 8 machines to 8.1 too. I noticed that the tool I installed to allow Windows Apps to run in Windowed mode still works, which means Microsoft has still consciously chosen not to support it, which I think is dumb. I do like that the 'Start button' is there to make it easier to show the Tiled Start Menu and that it works on either screen, but I'd have preferred to have that span both of my screens. I guess that would have confused some of the Apps, but it would still be very cool - I'd be able to see everything without scrolling.
The unified search is a definite improvement as well. If I search for GIMP and I find it's not installed, at least I get an immediate link to the webpage to download it. I like the right-click context menu on the start button to give some good shortcuts to useful features.
All in all it's a step forwards, but definitely not a very convincing one. I haven't seen any major feature improvements in the SDK either for Apps - most stuff seems focussed on performance improvements. They still have a lot of ground to cover.
BUT: I do much prefer developing for Windows 8 Apps than for any other App platform so far. I'm making nice progress with the Letter Trainer that I'm writing for my son. I have a first mode that trains him all the letters very efficiently until he gets them all right, with automatic repetition of letters he doesn't know immediately yet and nice visual feedback on his progress, resume now works properly too, it scales to any display size nicely, etc. I just need some finishing touches, adding a rewarding 'completed' state, an option to reset progress, and an option to select/add different users (it's already implemented in the logic), probably re-record the samples with a good mic (I used the laptop mic to get the first version of the samples), and add support for English.
Then I think I have a good enough first version for release, and perhaps try it out at his school if the teachers agree (they're working on letters there too right now). And then something like an option to record samples yourself would be a nice feature for the next version, as well as export the test data for analysis in Excel or something like that. Or make something built in, giving an overview of all users, shared weaknesses, etc.
We'll see. It's just a simple idea, but there's still a lot of time and effort that you can put in there, and techniques to test - glad I chose something this 'simple' to start with though, all my other ideas were far more complex.
Oh and that new default background ... man, that's retro to a point well-before personal computers even existed!