We leveled to 50 quickly with the intent of having an easy time running dungeons for gear, because normally, the players who reach max level quickly are all good players.

Due to legacy players, this is not the case. We're having a really hard time with legacy players who are 50, but extremely bad.

Roughly 2/3 of our tanks cannot hold threat off of the healer, let alone the DPS. They do not make any attempt to avoid damage. One of them kept using limit break on cooldown, not even during bosses.
About 1/3 of our healers just mash Cure, without using Cure II or Medica to adapt to the situation.

When we do get in parties with other good players, they always comment on how good the group is, and complain about their previous groups, so there are evidently a lot of really bad DPS, as well.

It's so bad that we're just leaving the dungeon and taking the 15 minute waiting period, because these players are performing so poorly that it isn't worth staying.