One of the biggest issues for tanks that I have seen is focusing on one target, maybe doing a flash here and there (but usually not) and not using any aoes. They just expect the dps to follow their numbers and keep their dps low enough. I have even had tanks tell me to not use bane on smn or fire 2 on blm because it's too hard because they don't want to deal with mobs that are not the one they have targeted currently. They want me to follow the 1 2 and 3 marks they took more time putting up then we used taking the mobs down.

It's not always the dps or heals, sometimes its just the tanks. And I have seen tanks that are severely under geared compared to myself and the other dps in 4 man dungeons hold hate flawlessly because they know how to utilize their skills properly so it's not always an ilvl issue.

You also have the tanks who just put on gear cause they can, the amount of tanks who come to dungeons in mnd, int, dex or str gear just so they can hit ilvl or even lvls 1, 5 and 8 gear lvl 30+ dungeons, is staggering. So it's not necessarily an ilvl issue alone.