I think that you could still work with a DPS threshold requirement per encounter for healers/tanks - healers would need to be way low as the "average" (since you have to go out of your way for DPS and things can easily go wrong), but tanks can be a bit higher. I think encouraging everyone to consider their DPS contribution is a good thing since the game is built that way in general, everyone can contribute some amount of group DPS. Gauging eHPS and damage mitigation runs into way more caveats than simple DPS would. I've actually thought about OP's idea a long time ago and was planning to post a concept thread for it, but OP's pretty much covered all the bases. My idea was that each encounter rewarded tokens based on your DPS contribution (bronze = average, silver = good, gold = great, platinum = high score for that encounter) with unique glamour/items available for each token set. You'd get your tokens at the end of the dungeon and if you have enough of a certain set then you can go to the Golden Saucer or something and exchange them for the stuff you want. Rotate in different glamour/items every patch to keep it fresh so that people keep aiming for high scores.

I dunno. I think there are certainly flaws with the idea of a score system and you'd probably need to fine tune a lot of things to get it to stick but it'd make things more competitive and fun. People often see competition as a hindrance, but (at least with my system) you'd only be really competing with other people at the Platinum tier, and that competition will help players improve - those who are uninterested are already uninterested so nothing would change for them.