Hey everyone! I'd like to take a minute to quickly run through a basic guide of what you should be doing to help out a tank in dungeons so you aren't the guy who's making the dungeon harder than it has to be - if you're new to ARR or MMOs in general, I highly recommend you read this! Keeping in line with what I just said, this is largely focused at lower-level dungeons.
Let's start with something that's essential: Do your best to attack the target the tank is currently attacking. Tanks have Flash to make sure that any slip-ups or occasional AoE attacks you throw out don't get you killed, but our MP is limited! If we start to use Flash too much, we have to stop using Savage Blade and switch to Riot Blade to regenerate our MP, which means we're generating less enmity (threat) than if we were to use Savage Blade. If there's a large group of MoBs, throwing DoTs on all six or seven of them and spamming AoEs is bound to cause trouble! If you're having trouble targeting the monster the tank is attacking, it's completely acceptable to ask him to mark the targets in order of which you should be killing them.
Secondly - and this is for Arcanists/Summoners - but please don't use your Topaz Carbuncle. When I'm tanking three-four (and more) MoBs, I'm not looking at each individual MoB to make sure they're all looking at me. I check my enemy list. If it shows one (or more) of the MoBs isn't attacking me, then I have to check to make sure it isn't attacking any of you guys, make sure the Carbuncle doesn't die and/or the enmity goes to another player in the dungeon, etc - long story short: As a tank, I want all the MoBs on me. I'll be using moves to increase my defense and enmity, and the healer can worry about the rest. If you're doing damage, focus on doing damage.
Lastly, and this relates back to my first point, but please follow the order in which the targets are marked. If I target the MoBs, it's to make the dungeon easier for everyone. You're not a hero because you killed a dungeon MoB by yourself. All you do is make it that much harder for me or any other tank to keep its aggro. And if I have to switch to a MoB marked #3 while the MoB marked #1 is getting hammered by everyone else, you bet I'm going to lose its aggro. Please, please, PLEASE follow the order in which targets are marked.
Other than that, have fun! Ask questions if you have to. But if you follow those three basic steps, you're already yalms ahead of what's considered a bad party member.