the best way i can see to make performance metrics happen, is to re-evaluate the way LFD works to 'assign' roles from the pool of candidates.
this has to deal with 3 problems - ranking and privilege, both are plagues on end-game. training, which _never_ happens in-game, and farming instances. probably some others.
ranking happens for tanks and healers all the time, it's probably unmanageable if you don't make the system work that way, since those roles are much harder and rely on experience more than gear. LFD, doesnt make the distinction, nor should it. with some exceptions, the tank/healer have a definite role to play. farming is easier to avoid if you adjust/balance the system for unweighted players, i.e. players who haven't run the instance at all.
training, ball, dropped. if class based questlines included CC, it wouldnt be an issue, i suppose. but endgame CC and mid-game CC, are broadly different. if it were an in-dungeon quest, and this would be punitive really, ie. cast sheep to not die, also does not work if players can just DPS around it. making an achievement for it, may be the only way to ensure the lesson sticks, but if it can be cheese'd with 5 ranged DPS instead of 3 players with CC, it will be.
having a requirement in the game, rely on a questline, is 'training' in it's dumbest form, but challenge events, i.e. shoot the marked target, sheep the moon target, all under a timer, is ultimately better at getting people used to the mechanics. problem then is, what do you reward people with who can manage it. a token to enter heroics, an achievement, some XP/money, or ??? ,preferably without jousting mechanics or something ill-concieved/evil
besides completion of the instance, and ilvl averages, and the elected role, there's not much to the way it decides to insert players in. i'm only sure it weighs players on 'co-operative' values, i.e. who kicks, who deserts/leaves, who's on ignore lists, and so on. and possibly it's a handicapping system, i.e. if you drop 4 instances that week, you have less chance to get in until the end of the week or until you complete 10 without dropping, sic, etc.
it's a black box, you queue, 40 minutes later you enter an instance. all we see is 4 options in LFD the role, and the elected guide. we don't see who's got the secondary roles, i.e. if ppl need to have CC/dispell/combat res, if they need to be social, if they have the right gear level, if they have the right spec, etc. it's mostly unwritten, and hidden from the player.
so, i've got 3 changes to improve LFD.
1) the role of dungeon guide, i.e. the party leader, should really be someone who had completed the instance the most times, overall, regardless of deaths or role. if that's the hunter, or the shaman, so be it, with a strong preference for the number of times they elected to be the guide and were successful as a guide in completing the instance. possibly even if they don't choose it from the box. i.e. after 20 runs, "something" has to stick.
that might be a good change to make. upside is, stronger guides, which cata heroics need, and it encourages people to re-run instances as a guide, but it also creates a class structure if a DG gets first preference when they enter into the LFD system, much like tanks and healers do, downside ? not all guides help the group or want to mark, they just tick the box because it's there. making the guide responsible for failure, may be the incentive to stop people being the guide, or rewarding them with extra rep/xp/gold, may be a better incentive to choose to be a guide.
overall, meh. good/decent guides are more random than DPS, and there's 3 of them.
2) there needs to be a category to fill for each instance 'type', so that you don't get a group who can't dispell the specific boss, if that mechanic is ultimately needed. a secondary queue system. or tertiary, really.
i'm pretty sure a 50k dps debuff is 'needed' territory. if the instance requires dispells, find a way to 'shortlist' or improve the odds for players that use it, rather than those that have the ability, but don't use it at all. this, then requires recording information during the instance, such as per-player monitoring of dispells, interrupts, deaths and after which stage of the instance this occurred.
it's impractical in a lot of ways, since 10 points higher in ilvl for the healer/tank/DPS will often faceroll those harder mechanics, along with a solid heal/tank/DPS strategy and CC management.and it particularly disadvantages players who don't have CC, over those that do.
3) don't queue players who haven't completed the instance, into an in-progress instance. or at least prefer those who have over those who haven't.
this one i left to last, since it's arguable. i don't mind when it happens, but it would be easier on everyone if there was a guarantee that people who dropped in, were able to catch up.
the downside is that early content would never drop anyone in at all, and the people that need gear, haven't done the instance, and won't get into an instance if the queues are quite large.