The bigger inbox can be very useful if, like me when I was a member, you have a lot of PM mail (I mainly donated for that). The blog is nice, and uh, I never bothered about the profile pictures or background images but they are a perk, why not.
The secret boards are interesting if you like to preview secret future stuff (not tooooo often, but with a decent frequence) or just spam a DCC.
And of course, you'll say bye to the top ads and get a coloured username. And you'll be helping the Site Admins to run up the place, don't forget.