Here's what I'd like to see happen with the duty finder:

When filling an in progress dungeon (lets say it's 2/4 and thus needs 2 members) do NOT have it pop up and fill 1 person at a time. What often happens is somebody will enter, go "oh, it's missing a tank/healer/whatever" and then drop, which creates sort of a "revolving door" dungeon that constantly cycles through members who enter and then immediately drop when the dungeon isn't full.

Instead, wait for the queue to fill the remaining members, then have it pop up for people to accept, and then put them in all at once. That way anyone that queues for a partial won't be getting tossed into a dungeon that only has 1 guy in it, and will instead be guaranteed a full group that will actually be able to finish without waiting. This also helps the people inside, as the group will fill instead of having people enter/drop nonstop.