Category colors are now shown next to an event box, which really neatens up the whole look of the summary view, and makes it easier to immediately focus on the stuff relevant to your current context (work, home, etc). I mostly use this lightning view and my events come from various different Google calendars, so I'm really chuffed with what is actually a relatively small change.
Several performance improvements, which seem to be quite obvious in the main calendar view. It definitely seems like my events are being pulled from Google a lot faster.
I don't use the task functionality, but considering some of the improvements I really wish someone would develop Google Notebook integration for lightning notes (or that Google would launch a standalone Notes product), similar to the Google Calendar provider because I'd love to use some of this stuff:
A full-on task mode with filtering similar to the calendar mode, and previews similar to the mail mode.
Easy conversion of mails, events and tasks into each other by just dropping items onto the mode toolbar buttons.
Why this plugin isn't yet a standard part of Thunderbird is beyond me..