Gear tends to matter much, but even more than that I value raiding experience. I'd definitely like to see which bosses/raids person has cleared and if he/she has done any of the hard modes.
Scoring mechanism itself should be something like many of the guild progression sites are using: the amount of points you get from the achievement gets lower as the time goes by & also larger cap when new tier gear is available. Something like:
* Naxx 10: base points 200
* Naxx 25: base points 300
* After ulduar is introduced will reward 50 points (constant)
* Ulduar 10: base points 2000
( the diminishing aint that big thing - the cap when tier changes is )
The result should be normalized so that clearing the highest tier raid should result close to max value.
Hard modes should be on separate score, but the scoring model could be about the same.