Unfortunately, the base assumption is that the tank in a 5man, is the group leader.
(If I ever wrote a series of things on tanking, I'd have one on basic group leadership from a tank point of view, because fair or not, it's the expected norm.)

These days there are a lot of convenience tanks. People who have a tank spec to get into dungeons fast. But just having a tank spec doesn't mean you're a true tank. In that light not every tank should be the groupleader by default. I played different roles to improve myself as a tank. As a dps I learned that there are a lot of tanks on meth. They keep jumping around for no apparant reason. As melee DPS that is hell. You can't get a hit in let alone make use of the opportunity attack. As ranged I don't care. Just nuke and get it over with. Ranged DPS is expected to do things? You're kidding right. The only thing I do on occasion is walk into flare core, but i will not interrupt a target that the tank is on and his skill has enough CD in between interrupts.

On the flip side, tank, if you don't know what you're doing, freaking SAY SO!

True story, a week or two ago I was helping a guildy do a non-twilight heroic for some gear, we got Throne of Tides. On the second boss (the one that drops purple void zones), the pug tank didn't kite him at all, we only won because all but one DPS (the guildy) was horrendously overgeared (the pug tank died, and the rogue tanked the rest of the fight).

I asked if it was the tank's first time in here, no answer. We offered to give him a rundown of the fights if he needed them, no answer. We get to the last fight, the tank isn't kiting anything, he obviously has no idea what to do, and was just running around like a chicken with it's head cut off. Oh I tore that guy a new one to the point my guildies were shocked as I'm usually Mr. Relaxed when it comes to n00bs in dungeons.

It is okay to not know a fight. We all had to start somewhere. Hell, its boarderline okay to bluff your way through not knowing a fight if you're heals or a DPS. It is never okay to bluff your way through not knowing a fight as the tank. Tanks *MUST* know boss mechanics! If you don't, just say so. Nobody is going to kick a tank for asking for pointers (might have someone drop group though, seen that happen before).

You don't have to go watch a YouTube video of the fight, although it would help if you'd run it a few times as DPS first, but when you have the entire group trying to freaking help you out and you absolutely refuse to even listen to them, then good God, give up tanking.

The main reason I don't tank very often is this right here. Yeah, it's pretty easy to find guides to every *boss fight* in a heroic dungeon. But trash pulls? Good freaking luck. Even in early Cata, when places like Vortex Pinnacle had trash pulls ten times more lethal than all of the bosses put together, there were essentially NO pull guides, and just running through the dungeon a few times as DPS might teach you, "Hey, that one's a caster" but isn't necessarily going to teach you, "Okay, LOS pull that one, then skull the one on the back-left, because he's a healer." Frustrating.

@edy I totally believe in Qing as dps the first few times in a new dungeon. if you plan on tankin/healin ask for pointers. at least to save yourself a scolding by frothing strangers.