Proper use of line of sight can be just as important to tanking as threat rotations. Inproper use will get you killed or your healer killed. There are two main types of LoS that you will need to be worried about. The first one is Pull LoS.
Pull LoS: Pull LoS is exactly what it sounds like. You pull a mob or group of mobs and use LoS so that they all run to you. This is very important to AoE tanking and even non AOE situations.
This method is particularly effective against two types of mobs. The first is ranged mobs. Ranged mobs are more than happy to sit at distance and try to eat your face. They are also more than happy to eat your healer's face after that first big heal. We have tools for a single ranged CASTER mob such as Heroic Throw. If there are more than one caster mob, or even a single non-caster ranged mob (think the Hunters in Halls of Lightning or the Spear throwers in Utgarrde Pinacle) LOS is the only way to pull them back (outside of charging in like a mad man).
Obviously, charging in is nice, but it is not always an option. A great example of perfect use of LoS in this situation is the last few trash pulls before King Ymiron in UP. There is a group of 4 with two casters. There is also a patrol that walks right through the area. Waiting for the patrol and LoS the large group back to the previous room is very helpful. If not, it can become a mess as the patrol fears and you may pick up the abomination as well. Now onto our second form of LoS....
Heal LoS: This is staying in LoS of the healer. If you run out of LoS against some dangerous mobs, you may be dead before the healer can even get there. A good tank know where their healer is at all time. For the most part, a dead tank mean a dead group. Obviously, the Healer should realize that you are out of LoS if the situation occurs. It is both of your responsibilities to find each other. Sometimes getting out of LoS in unavoidable, such as being feared off by a mob. You can get out of the fear, but your healer may not be able to.
On a related note, staying in LoS of a healer is just as important as staying in healing distance of the healer. If your healer is drinking, and you run up two flights of stairs and aggro the next group, it is your own damn fault if you die. Keep an eye on you healer's mana bar at all times. Distance problems can also occur when a tank is too far ahead and then charges. Being 50 yards away from your healer may not be a good thing. Keep all of these things in mind while chain pulling and it will lead to less deaths.
Remember, living tanks are happy tanks, and happy tanks come from living healers :).