As an attendee, I'll be honest and say I was there for the free stuff. It was a bit disappointing we only get watches this year. Would've liked a new tablet or something. But oh well. Yes, yes its free, but I/O has always been known for gadgets.
Moving on though, I do find L to be a much needed polish of Android. We've been sitting on the same UI since 4.0, which is very nice, but lacking features. The custom ROM scene has really been pushing the limits in terms of what Android can do, but overall we haven't seen any major OS update since 4.1. Most of the rest of the updates are separate apps that bring in more functionality, but in the end we aren't getting more controls and toggles and features in the OS itself.
The new interface does leave some questions though. Are slide out menus to stay? Or are they going away? We spent so much time bringing them in, and while I'm not a huge fan, I've adopted. What's worse is the Google+ app right now where I have to reach to the top to interact with the app and I have to go back to the main page to switch to a different page as a result of the lack of a slide out menu.