I am going to start a new series on basics of raiding. There are a lot of raid mechanics that has been used in previous expansions that are being brought back in Cata. With how many new players in LK tactics of older raids are not known to them. I am hoping this series will help you out when raiding. Know that not all of these apply to every boss fight. These are just guidelines that tend to repeat throughout the game.
Don't stand in front of the boss.
Unless you are the tank you shouldn't be in front of the boss. Many times bosses will have a cleave or cone effect that will cause a lot of damage to anything in front of it. Tanks have stats to help decrease this damage and the healers are focusing on them. A stray DPS standing in front will likely get one shot.
Dragons tail swipe so don't stand behind them.
Many of the dragon type bosses have a tail swipe which can cause a lot of damage and sometimes do a knock back. If you are a melee DPS don't stand any further back than the dragons hind leg. If you are ranged or a healer stand off to the sides or at the very least stand near max range behind the dragon.
Don't stand in stuff on the ground.
If you see the ground rumbling or some kind of fire or slime on the ground don't stand in it. Sometimes there is some sort of warning that something bad will be on the ground and other times it will just appear. Move away from these areas asap. Another thing is stuff falling from above. If you see stuff dropping on you then move out of the way. There are a few instances where you shouldn't move out of stuff on the ground like Shade of Aran in Kara's fire but those are few and far between. 95% of stuff on the ground should be moved away from.
You can't DPS or Heal when you are dead.
Not all fights are a DPS race. Some involve so much movement and environment awareness that slow and steady is the best option. If you are putting total focus on your rotation then you are not paying attention to the fire ball heading towards you. An old term for DPS was Glass Cannon. You can deal out a lot of damage but you can take very little. Blizz uses a lot of different mechanics and some are to throw people off of what they think is normal. Think of Heigan the Unclean in Naxx. Before you can DPS you have to master the dance so you wont die.
If you have to click something be there before hand.
This is another DPS flaw. There are some fights like Magtheridon where you need to be at a certain location to click on an object during the fight. When using an addon like DBM go to that location early enough so you are standing by the click for a few seconds at least. Don't try to DPS as much as you can and cut the travel time to the item short. You never know when there will be a knock back or stuff on the ground you have to run around causing you to take more time to get to the click.
If you are to CC then that is your main focus.
If you are in a fight where you need to CC a mob, such as the Moroes fight in Kara, then your main goal is to keep that mob CCed. You doing slightly less DPS will make up for a mob running around smacking healers and wiping the raid.
Not all bosses have argo tables.
I mentioned Shade of Aran before and I will again here. There are some bosses that are not tanked like how we are used to. Some pick a random person and they have no threat level. On these types of bosses the DPS can do everything they can to top out their DPS so long as they pay attention to other game mechanics such as stuff on the floor.
If you are sucked in you need to get out fast.
General rule is if a boss is able to suck you to them you need to get away from them as soon as you can. We have seen this with Shade of Aran in Kara, in Blackrock Caverns, and Asaad in Vortex Pinnacle.
One of the best things you can do when learning a boss fight is to check out web pages or YouTube channels that have write ups and videos. Don't just read one write up about a fight because many times they will list only one tactic and usually have a lot of numbers and theory crafting which is just too much info for the first time raider in an instance. Watch a few videos and once you have the fight down pat then you can go back to theory crafting of the fight.