A lot of people seem to have the idea that the main tank should also be the main assist, the guy everyone assists to find the mob that people should do damage to. This isn't true, in fact this is probably the worst way of doing things.

The main tank and the main assist should always be 2 different people, generally a damage dealer should play the part of main assist.

This is due to the fact that as a tank you will never be targeting the same mob all the time. You will need to constant rotate between each of the mobs on you to keep building threat on each of them. If you sit there attacking the one mob that everyone is attacking as soon as a heal goes on every one of the other mobs will run straight at your priest, and remember what we said about protecting your priest? And if you're acting as main assist as a tank then people could have a different target depending on when they decided to assist you, and there's nothing worse than trying to keep 2 mobs on you that are being spammed with high damage attacks.

The way that I find a lot easier is if one of the other members of the party is the main assist, and everyone has their assist macros set to them even you) This way you wont have your party complaining that you keep switching target and they don't know who to attack.