Alright guys, Sartharion is a serious boss, and it's going to take a serious strategy to beat him. I drew up a strat for taking him out, and I want everyone to follow it to TO THE LETTER.
The encounter starts with the raid at one end of a rectangular area surrounded by lava, with Sartharion at the other. I'll be referring to where the raid starts as the "bottom" of the area, with "left" and right" being relative to this perspective.

As soon as the battle starts, the tank will pull Sartharion over to the right, rotating him 90 degrees. The tank (represented here by a bear druid) will be backed up to just before the lava, and roughly centered between the top and bottom of the area.

Here's what that looks like from the tank's perspective:

That's all there is to the initial pull. Positioning is important, but if we're a little off, it should be easy to correct mid-fight.

Melee DPS (represented here by two cat druids) will be on Sartharion's side, a bit spread out. They MUST avoid being near Sartharion's face, as he has a nasty cleave. Being near his tail will make it easier to hit Sartharion when he rotates (more on that later) and dodge lava walls (more on that later). Thankfully, Sartharion has a HUGE hitbox, so you can be a bit flexible with the positioning.

Ranged DPS (represented here by a moonkin druid) will be a little ways back from Sartharion. They should err a little more towards Sartharion's tail, because...

Healers (represented here by a restoration druid) will be in the same row as ranged DPS, more towards his head. Again, don't get too close, or his Cleave will destroy you.
Here's what the positioning will look like for most of the fight, from the DPS's perspective:

Now, the major thing about the positioning is that the platform is divided up into five or so zones:

Lava walls will occasionally rise up from either the left or the right, and slowly sweep across every other zone. This is preceded by an emote about Lava Surging around the Platform or something.
When they rise from the right:

...they'll cover the even zones (2n). If we're in position, the tank will not have to move, and everyone else (possibly even the melee, depending on Sartharion's position and how close to the edge of his hitbox they are) will have to run up to Sartharion's belly to give him a hug. Er, avoid the lava wall, like so:

Until the main tank has experience with exactly where the safe zone is, he should try to keep an eye behind him to make sure the first lava wall doesn't catch him by surprise.
When they rise from the left:

...they've cover the odd zones (2n-1). If we're not in position, THAT'S A FIFTY DKP MINUS. Otherwise, the tank will have to strafe right, the melee will have to back off, and the ranged classes will be safe. That'll look like this:

Notice that Sartharion has rotated. It's possible the melee DPS will be able to clump up near his tail and continue to hit him (he doesn't have a tail swipe, apparently). (EDIT: Sartharion does have a tail swipe that stuns.) If it's possible, go for it, otherwise chill out until the lava wall has passed and the tank has rotated him back into position. From the tank's perspective, it'll look like this:

Try to keep your camera zoomed out so you can see the lava wall forming on the horizon. Even if it's hard to see behind Sartharion, you should be able to simply look back and to your left whenever the lava emotes to see if a wall is forming there or not. If it isn't, you'll have to move!
Also, occasionally adds will spawn in the raid. These will look like fire elementals, exactly like this:

The off-tank will need to pick these up right away, followed by the DPS burning them down with AoE.

The off-tank must continue to watch for lava walls during this period. It would be nasty enough to get hit by a lava wall while tanking the fire elemental pack, but even worse, the walls will turn a normal marshmallow seal...
...a burnt marshmallow seal! Burnt marshmallows deal 400% more damage, which will probably wipe us if a whole pack gets hit by it. Therefore, the off-tank must do whatever it takes to get the elementals out of the path of the lava waves. He/she should probably keep them away from the edges of the area, to avoid surprise lava walls. Sartharion will normally summon 2-4 elementals at a time; however, when his HP gets low enough, he will start summoning more. When he does so, healers will need to give extra love to the off-tank.
...also Sartharion will occasionally lob slow-moving fireballs / meteors at a random person in the raid. You can and should dodge these. It's also a good reason to stay spread out, so he doesn't constantly drop meteors on the giant raider clump. Think Void Reaver, but a lot less frequent. I just don't know which of my stuffed animals best represents an AoE meteor.
Follow these steps, and we should be taking a success screenshot this very night!