Things tanks SHOULDN'T do:

1). Stack ALL of the defensive cooldowns before even pulling the first enemy (while running up to an enemy). This means the tank is clearly using a macro that fires everything on cooldown.
2). Balls-to-the-Wall on Pull #1 without even trying to gauge how good the healer is.
3). Not caring about which enemy is the most dangerous/should be killed first. For example, the Lv73 dungeon features 1 enemy in the middle of the first few packs that buffs the enemies around it. Its buff spell can be stopped but it needs to be done QUICK. Most tanks don't care and so these groups take absolutely forever to kill because the mobs got buffed.
4). A tank should have at least 50% more health than the DPS. If the DPS has 50k, the tank should have at least 75k. If they don't, they probably have terrible gear. Of course, if the DPS is level sync'd and the tank ain't, then it might be more like 30%. If the tank has about the same max health as the DPS/heals... something is very very wrong. Broken gear, bad gear, etc. I have actually seen this a few times. It's a nightmare to heal such tanks.
5). A tank who throws their weapon and just mashes sprint and keeps running. Should use at least one AoE to make sure you have the mobs.
6). Not using Mitigation on Tank Busters. I don't know about WAR, DRK, or GNB, but I know PLD has Shelltron. I've also seen lots of PLDs who don't care to save it for tankbusters and end up eating the full tankbuster with no mitigation.
7). Multipull after wiping on the very same Multipull (I've had tanks do this, we wipe because someone decided they should be pulling 2+ groups in some <Lv40 dungeon when I have zero instant casts, we wipe, and they.......do it again and we wipe again)

A Tank SHOULD:

1). Instead of dumping all of their cooldowns at once, space them out. If they have multiple defensives, use the best one once you're done moving and all of the mobs come to them. Then when that one wears off, pop the second one.
2). Know the fights, and know when to pop mitigation abilities (esp for Tank Busters).
3). Use Group Defense abilities (Path of Arms, Divine Veil, Clemency, etc)
4). Pull the first group, single to gauge the healer's experience and talent before going crazy on multipulls (or better yet, ask).
5). Know what healers can do what at what levels. For example, a <Lv50 WHM has no Instant-Cast Heals. Lv50 WHMs only have Benediction as their only Instant Heal. They don't get Solace until.... 52? 54? something like that? Keep this in mind when doing these huge pulls.