* prefetching messages. When you load a page that contains a list of messages, Gmail will fetch them in advance so that when you need them, they'll be available instantly. Google said that Gmail will have a completely new JavaScript architecture that will bring a much better performance.
* a new contact manager that will be shared with other Google apps (Google Docs, Google Calendar etc.)
* integrating features from other Google products (an example of an existing feature was the "Open as a Google Document" links placed next to Microsoft Office attachments).
Google didn't mention anything about an interface change, but a recent discovery showed that Gmail also prepares a new UI. Maybe the features I anticipated last month will still make their way in this new version of Gmail.
Related: Google Analyst Day 2007 @ YouTube
Update (a day later): Gmail 2.0 is here.