My Personal Gripe.

Non-Tanks:

You will hear me blather on and on about these nasty, tank bashing, inconsiderate people.

There are those out there that are not a tanking class or have never played their tanking spec and yet full understand the needs of a tank. These people get a big thumbs up, many tank brown points (to be used against any genuine mistakes), and an honorary tank membership from me. As such they do not fall into this category.

To my knowledge there are two category's of non-tank, the first being the Dps Junky.
Sounds familiar?
It should as there are so many of them, the power mad mage that just doesn't want to let you pull or that wont understand that big crits mean big threat, the over eager rogue that attacks your target right before you can hit it, the crazy hunter with the pet on growl, the crit happy Boomkin that just wont slow down, and my favorite ... the dps warrior who thinks he's the tank.

The second is the "Healing causes threat?" healer. You will hear me later go on about treating your healer with respect, but the reverse is also true. You will find healers that slap a big heal on you right after you Bloodrage and wonder why all the mobs ran right for them. The healer that even when space allows stands right next to the combat healing his butt off not realizing that getting a little more space increases the amount of threat needed to take a mob off you.

Dealing with the Non-tank:

What should we do when we encounter these people?
Well the thing we don't do is fly off the handle calling them noobs or start the whole L2P thing. this just makes you look the complete prick and destroys any respect the party had for you to start with.
What we do is wait until the fight (or wipe) is over then start the conversation with...
"If you don't mind I have a suggestion." or similar phrase (and if it was a wipe you will be amazed at how readily people will listen).
You then continue with ...
"Well if you were to (Insert appropriate piece of advice here) it would help me maintain aggro."
Never ever accuse people ... especially if they did wrong, this will make them defensive and less likely to accept advice. By all means point out their mistakes, but in a nice way and add a smilie face to take the sting out of it.

Ok you've been nice, you've made your suggestions and they still repeat their mistakes?
Now we have a problem, as I see it you have two options 1) Put up with it and do your best then avoid grouping with them again, 2) Give them an objective lesson in aggro management.
No.2 Means letting them take the aggro, then letting them die, recovering it and finishing the pull in full control. Thus showing them (with a large repair bill) that they "done did wrong". This requires you to have full confidence in your ability to regain control of the situation, and be a bit spiteful.

Warning:

This is not me giving you a license to boss everyone around, and make d@mn sure that it was them that was at fault and not you (otherwise you just look like a complete @sshat).

Remember that if they realize their mistake and apologize you need not go through any of the Dealing with the non-tank steps you just keep quiet, give 'em some tank brownie points for owning up to it and check another tick on their honorary tank membership form.

Also remember that making multiple but different mistakes is allowed. Often people wont know that something is done right until they do it wrong.