The level-difference idea has a few flaws:
The first is Murder Shot and buff-steal builds which pretty much depend on alts. I'd prefer not to need to make a level 90 throwaway character for MS to hit more than a wet noodle.
The second is party defensibility. You're partied with a guy who's level 90 while a good portion of the party is at 85. Some 100 comes up and kills him and you can't do anything about it. Or make it 35/40 and s level 50, and so on. Make the victim the priest or tank and it's effectively a TPK.