I know the title is a bit provocative, but this is an attempt to connect with DPS, so that you might understand what tanks are dealing with.
The best way to start this thread is probably with an olive-branch. I'm not criticizing every dps, I'm trying to talk to the ones who want to learn, and don't already know.
I thank good DPS in groups, and I thank BLMs for sleeping mobs. I know that most tanks are jerks and that only brings out the jerk in other people. We're not the boss of you, and can't tell how how to play, but we're sort of the Leader, no matter who has the icon, and its expected that we're familiar with each enemy and know what to do. If everyone (goes both ways, and healers included too) could just be nicer, everyone would have a lot more fun.
Tanks can't say this each time they enter an instance because A) people don't want to wait as he talks B) people start to get crabby, thinking they're being accused.
I'm actually a fine tank, I hold hate, I survive, I move out of every red bad-spot that I can and I move to intercept adds where it's appropriate (it's almost always appropriate, some bosses are an exception).
This is really written from a paladin's perspective, as I haven't played Marauder since 1.2 and can't really speak for it with authority.
Our AOEs (Flash, at least) are not meant to tank off of you
I've tried, out of curiosity, to just spam flash, and it will not hold hate off of good DPS. Another reason that it won't is because our MP is not unlimited and we have to break into a single target combo for 3~ seconds to recover a little more than the cost of one flash.
Flash is a tool for snap agro, it's to get the mob's first attention on me. It's then used throughout the fight to keep their attention on me. It is not meant so that you can attack whatever target you like.
Flash is also meant to establish hate off the healer, and to keep it off the healer. In a fight with many mobs, it must be used quite a bit, but the only people who should be putting hate on mobs I'm not targeting, most of the time, is myself and the healer. Obviously, boss fights are different.
Attack our marked targets
Kinda goes along with the first, but a good blm or healer (if they can) is probably sleeping targets. A good tank is marking his target so that everyone in the group knows what to attack. I don't want to use Provoke for you, I prefer to save that for the healer, or if I screw up. With rare exception, the enemy group isn't going to die any faster if you attack your target or mine, and actually, a little slower. If everyone concentrates on one target, our debuffs work together.
We don't like it when you pull
I know that some tanks move at a snail's crawl and I know they're annoying (hey, I dps/heal too) but some perhaps they're doing something, like equipping a new piece of gear, or they're waiting on something. Perhaps they see something you don't, or want to do the pull a particular way.
I can offer no excuse for the especially slow tanks, but just because you're in front of the tank is not a valid reason to pull. This especially holds true for casters. You can pretty fairly sprint at the expense of your TP because TP isn't as critical for you (I haven't played a caster since 1.0 besides my conjurer at 20). Casters can get to the target before me, and still, you're not helping anyone by pulling.
Remember though, whatever reason a tank may be pulling slow, it's on his shoulders to clean up a mess if it happens. There may be some reason he's not ready. Hey, who knows, his grandma may have caught fire.
Don't attack til the tank stops moving
This may not hold true at endgame, where timers might be a concern, but you can give the tank a second to position things. I don't like to type before each pull, but there are a lot of pulls I like to line-of-sight. Sleeps are okay, but preferably save those things until everything is in position. I still want to Flash sleeping things.
Use your own marking macros
Do not use [attack1+], because this is ultimately the tank's decision what to attack, but use a macro to mark targets for sleeps and such. I know that what I know what the healer is sleeping, i'll attack something else so that the sleeps last as long as possible.
Diminishing Returns
It seems that all of FFXIV's "enfeebles" operate on diminishing returns, this means that each time you use it on the mob, it will become more and more resistant, until it is eventually immune. After a time, the immunity resets, but things usually don't live that long. If you wake a sleeping mob, someone cannot simply resleep it, the sleep will last less time, and very quickly become immune.
Enmity Meter / Hate Bar (Thanks Rilr)
Below each character's icon in the party list is a small thing that looks like an hp bar. This tracks each member's enmity towards your current target. Sometimes you might desire to back off a little bit if you are riding up on a tank's hate.
Ask if anyone is new (Thanks Maxxus)
If you've done a dungeon, ask if anyone is new. Be willing to give advice. I did my first Stone Vigil last night, and a nice Bard explained the boss fights to me. He also told me about a particular pull I'd want to pull to a side-room. It saves everyone the hassle and potential wipe caused by new agro.
Once the tank knows a fight, it's his job to explain it, by default, but everyone is new sometimes.
Be kind
For every **** tank you meet, he's met a dozen ***hole dps and healers. After a time, people enter an instance naturally on the defensive. It's not pleasant, but it's just how it goes. Break the ice. If **** hits the fan and people still pull through, commend everyone. Even if it's just to say {Good job!}