If one examines the privileges earned above 100 reputation, they are split into two camps:
Privileges that are dangerous in the hands of someone unfamiliar with the domain.
Privileges that are simply rewards for good participation. These include:
My suggestion is that these passive privileges (and any future privileges of the same type) should be made available across all StackExchange accounts as soon as the necessary reputation is earned on any of the sites.
The fact that a user has earned sufficient reputation in another domain indicates they are probably of good character and a responsible user of the site. Hence, they should be trusted across all sites with non-dangerous privileges they've already earned.
I appreciate this may be difficult or impossible to implement as the current system is tightly wedded to one's per-site reputation, but I thought I'd raise the idea anyway.