The group finder should have/show an average wait time until your Que pops, so that the player can have A since of when the Que will pop and or that the group finder is working.
Since Tanks and Healers are always the most portions needed for the group finder there should be A bonus exp or gold or both or whatever you think is best, for players that actually get filled in the position of the Tank or Healer.
Since anyone class can be A decent Tank or Healer as long as they have the correct weapon equipped, and the knowledge to do so. this however is still A problem while leveling because most people want to level as fast as possible and usually go for the damage and will just wait to get to max level then will go back and level the appropriate weapon to Tank or Heal. Leveling dungeons should be able to be completed with the abilities just from the weapons alone, problem still being that you would have to level the weapon still.
Instead of forcing people to level curtain weapons to make Que go faster maybe there should be like A buddy system of sorts that lets you choose one weapon style in your inventory that levels with your main weapon so that you can switch between roles for dungeons, or something along those lines to make the dungeon finder relevant.