One of the most subtle aspects of tanking and positioning a boss is its "hit box." You can melee hit an opponent in WoW without actually touching your graphical character to your opponent's graphical character. Each character has a range around it that describes where the character is (versus where it appears to be). Think of it as a circle around the graphical representation. If your circle interacts with the boss's circle, you can hit it with a melee strike; obviously, it can also hit you.
Bosses tend to have very large hit boxes. The large hit box allows more melee characters to be in range of the boss without necessarily standing on one another's shoulders. Not all hit boxes are created equal. The hit box on Magmaw is fairly tight, for example, while it seems like Chimaeron has a gigantic hit box that extends virtually halfway across the room.
Moving a boss
The size of that hit box matters to a tank who needs to move a boss. A boss only moves if it's not casting and the target is out of its hit box. If you simply strafe from left to right, you may find yourself never leaving the extant hit box. If you don't move out of that hit box, the boss won't need to move.
You'll sometimes hear tanks say things like "this boss moves like a bus" or "he doesn't like to move." That's usually because of an enormous hit box. The tank really needs to hustle to get the boss to move anywhere at all.
During the first fight in Throne of Four Winds, for example, you'll fight the boss named Anshal. He puts green circles on the ground that the tank must kite the boss away from. During portions of the encounter, however, Anshal gets really really big, and his hit box gets equally bigger. You have to be at nearly opposite corners of the platform to move him at all.
These factors can prove challenging for tanks. Kiting a boss if relatively simple, but when the hit box is gigantic, they must move in a faster, more elegant fashion to put the boss where they'd like.
Cleave and cone
A frontal cleave attack and a cone attack are two typical, iconic moves for dragons (and part of why avoiding cleaves and cones are often called "dragon positioning"). A frontal cleave means that anyone standing near the tank is going to get hit by that melee attack, and a cone attack means that anyone standing in front of the boss is likely to get hit, no matter the range.
When a boss has a cleave or a cone, the tank must face the boss away from the raid. This is can be a problem, though, since it creates more distance between the tank and the healers. If the tank must move a boss away from the healers, there can be an interruption in heals because the cleave and cone has already created a mandatory distance.
A close cousin to the cleave and cone is the "tail swipe." Even if you're not facing a dragon per se, some bosses still issue an attack directly behind them. This is no worry for a tank or ranged DPSers, of course, but it's a huge pain in the neck for melee. (By default, melee attack their target from the rear.)
In a perfect world, your tank is at the 12 o'clock position of a boss. Melee are at 6 o'clock. Ranged DPS and healers are scattered around the boss at a decent range. Boss mechanics like void zones and fire mean you need to move to avoid damage.
In a perfect world where a boss doesn't demand special positioning, a tank should always strive to keep the boss's head away from the ranged DPS and healers. Don't go crazy to make that happen if the boss doesn't have a cleave or cone, but in general, folks should be away from the tank.
The reason you give a tank a wide berth on the floor is mostly habit and reaction time. By habit, keep the way clear so that no cleaves or cones accidentally mow down a non-tank. Reaction time is a bigger deal. If something horrible happens and the tank loses aggro, a little distance between the hit box and its next target provides time for the tank to taunt and get threat back. The further the boss has to walk to kill its next target, the more time the tank has to react.
Hopefully, these rules make things a little easier on new tanks and raid leaders. While every boss has different mechanics, these are the basic concepts that drive positioning.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.