I love the reputation privileges: it is fun seeing that Stack Overflow recognizes your effort by giving you nice little toys. So I would like to suggest adding the ability to add a gravatar as a reputation privilege instead of it being unlocked by default.
I don't think this is a valuable change.
The whole purpose of Globally Recognized AVATARs is that wherever you go you can display the same avatar. Making that a rep privilege flies in the face of that, as BoltClock's a Unicorn mentioned.
Re: the "goatse problem", Stack Exchange already requests G-rated gravatars. I've only seen one inappropriate avatar in all the time I've spent on the network (a stick figure with a penis on SF), and that issue was "dealt with" quickly.
Just my $3.50
Fun you're asking about seem to be pretty nicely covered by existing expanded flair privilege.
Just hover over gravatars of 1K+ rep users to find out how much fun they are allowed to have.
If that's not enough then I don't know what is.
...except for maybe an option to show combined flair instead of local but that's rather minor.