The big change with this version is the oplog tailing instead of the poll and diff which has dramatic performance improvements over the previous version. More details at the wiki: https://github.com/meteor/meteor/wiki/Oplog-Observe-Driver
I would have expected Meteor UI to be in 0.7, but that not a big deal and that will probably be in the next version.
We push them out first as prereleases (which you can run by passing a --release flag to meteor, and it will automatically download and install everything) and announce them to meteor-core and see what people think. In this case both oplog tailing and Meteor UI were out as prereleases, but there was more feedback and more feature requests for Meteor UI, so oplog tailing ended up winning the race.