This signature from the official Blizzard forums sums it up nicely:

Your $14.99 says you can play however you want.
But the rest of the raid’s $359.76 says to know your role

When you play WoW alone, solo, as if it were a single player game, I am all for doing whatever you wish. But when it comes time to raid, Courtesy is King. 24 other people are all depending on you to do what you must to see that this is a successful raid. Some laugh at the use of the word responsibility, but that is what raiding is, even for the casual crowd.

1. Be on time. If the raid starts at 8, be online at 7:30 and double checking your gear for the evening.

2. Double check your gear. We all do things other than raid. But be sure you are raid ready at pull time. This means PVP gear stowed, no fishing poles, no tournament lances.

3. Check your spec. Are you PVP specced? If you are one of several classes (like Warlocks) there are only so many of a certain spec that are useful in a raid, have you checked with the others to see what specs they are? Unless your raid leader has made it clear it is okay, be sure to not double up. Also be prepped to be either of your specs for the raid. Dual Specs make you valuable to a raid group.

4. Reagents, shards, food. The shame rank is especially useful here. If a Warlock shows up to a raid without soul shards… SHAME. It is simply inexcusable to show up to a raid without a vital part of your class’ equipment. The tank doesn’t show up without his weapon, you don’t show up without yours. Reagents are needed and yeah, the cost sucks, but hey, that utility you prove with that reagent is part of why you get the invite. Everyone in the raid has some form of cost. As for food, bring stat food, regular food, or feasts, I don’t care, but do not rely on Mage food. The one time you really need it, is the one time a Mage won’t be on the raid.

5. Repair. Nobody likes the guy who has to leave the raid 15 minutes in to go repair. Do it before, when you check your reagents. Repairs bots are awesome for this.

6. Mods. Make sure all your mods are working and up to date. Some casual guilds don’t require mods, but Deadly Boss Mod and Omen are so helpful and wonderful you should use them if possible.

7. Boss Strategies. Have you been to this instance before? Have you done the role you are about to do for this instance? Be sure to brush up.

8. Know YOUR ROLE. Dual specs allow raid leaders to pick and choose people, not only based on their ability to do their main role, but also their ability to do their secondary role. If you are a Holy/Shadow priest, know both roles and play the one the Raid Leader asks you to. Sure DPS is easier and more fun, but without heals, the raid doesn’t make and nobody gets purples.

9. Learn to Role with it. Yes I did mean Role. This is best explained with a story.
Awesome guild goes into Naxx 25 for the first time. Gets to the first boss. Starts fighting. Right before the first insect swarm the warrior tank goes down. DK#1 flips on frost presence, taunts, and takes off running along the wall. Cheer! The raid is saved. Gets to other side, Off tank does his job with the add and here we go again.
Suddenly DK#1, hero of the hour, goes down. DK#2 flips to frost presence, taunts, and then takes off around the circle. (We have the boss at half health.) Someone laughs and says, hey, third and fourth DK, be ready. Not too long and the second DK goes down, and sure enough DK#3 took the raid leader seriously and immediately shifted to his new role, frost, taunt and run. In the real world this is called adaptive survival. You saw a need and shifted to fill it.
Back to the story. DK#4 gets his chance to be a hero, and the boss is getting pretty low. Suddenly the off tank dies and there is an add slaughtering the raid. The two warlocks, as a team, start scorching bursting the add to get aggro and kiting him around the room into the frost mage’s aoes, effectively locking them down.
Then DK#4 goes down, with the boss at 2%. Bad-ass mr. pvp rogue steps in, pops evasion and tanks the boss just long enough for the mages and warlocks to burn him down. By this point all the ret pallys and elemental shamans are healing. Anu goes down with 8 of 25 raid members standing. Now any hard core raiding group would hate to be a part of this story. But for us, it is the thing of Legends and we talk about it all the time. If a raid leader shouts out in the middle of a raid to switch to x. Know your class well enough to switch.

10. Have fun. If you aren’t having fun, find a way to have fun. Raids, especially progression raids can get wipe-y and tense. A good raid leader should call the raid before shouting starts, but tempers can flare easily. Remember this is still a game, and your guildies are your friends.