I find the "rebootless" kernel upgrades to be one of the most interesting ideas, for a few reasons.
Firstly, exactly what it says on the tin - being able to upgrade a kernel without having to wait for my user session to start up etc. would be rather nice.
Secondly, a potential alternate hibernation mechanism that might bypass some drawbacks of the current options (swsusp and tuxonice) - specifically, the whole 'you must be running the same kernel' thing.