I loved what I saw about JB. The app encryption is not getting as much attention as it deserves. It's a huge step towards reliability, will help developers protect their IP and get rewarded, will weed out malware that has used repackaged apps as vehicles in the past.
"Butter" is also a good initiative, I had assumed that this sort of measures were already taken with ICS, but it is commendable that Google works continuously on improving Android efficiency.
The voice search is evolving nicely and I expect it to overshadow third-party apps such as S-voice. It seems like a natural development of Google search, towards two-way speech and context recognition built in. Off-line STT is also nice to have.
There was so much in the keynote that I got overwhelmed -- Google+ deserves an honorable mention, really digging the party mode.
All the devices announced -- OMG! Nexus 7 looks like a certain buy for me; the $300 Q thing not as likely. Really loved the Project Glass show -- things like this make Google the most amazing tech company.