In my time as what I would consider a hardcore raider (albeit 9 hours a week), I have picked up some things that differentiate the people who are effective raiders and the people who want to make everybody pull their hair out. I wanted to take today to point out those things. These things are pretty much regardless of what raiding tier or expansion we are in but this will be more focused on guild raids and not PUGs.
1) Prepare - There is no excuse for not coming prepared to a raid. You should have flask to last the entire time allotted. You should have reagents for buffs. You should have your gear gemmed and enchanted with the best possible enchants/gems for you. You should have a basic understanding of fight mechanics and any proper addons necessary.
2) Readiness - When you first come into a raiding guild, you may not be pulled in for every fight especially depending on where your gear is. You may come in for farming fights and then be put on the bench for progression fights. One of the best ways to show you are willing to do what it takes is to step up, be in Vent, and be ready when they say they need you.
3) Optimize - This goes for a few different things. One, your gaming setup should be to the point where you are not constantly disconnecting during 10 or 25 man raiding, depending on what you are doing. You may not be able to run your settings on Ultra but can run them on Good. That is fine. It takes being in a raid setting to get those video settings right. Optimization is also looking at what you have and build the best possible combination. If you have a PvE and a PvP piece of gear that are both the same iLevel, you should be using PvE gear in most cases (tanks being the notable exception). Programs like Rawr can help with this optimization.
4) Research - If you are coming in for a new fight, research the mechanics. Seeing any video of a Professor Putricide kill will tell you that you should not get hit by Malleable Goo. In addition, you should research and know your rotation, what to do in certain circumstances, and how you can be most effective. Your gear may not make it where you will top the damage charts, but you should be competitive.
5) Learn - Learning can come from two ways - education or experience. In this case, both are invaluable. If you see someone who is the same class as you and is outperforming you, ask for help and learn from them. The other way you can learn is from your mistakes. Admit you were wrong to yourself or your raid if you need to and then correct it. If you are not certain how to correct it, ask. Every raider makes mistakes from time to time, but admitting it and learning from it is the only way you can get better.
6) Observe - Raid awareness is key to being an effective raider. If you know that HM Sindy is about to pull in, you need to make sure your stacks of Unchained Magic clear before you get pulled in so you don't explode the raid. Or if you get unbound plague during phase 3 of HM Professor, you need to possibly back off and purposely die to get rid of the plague out of melee, where it could affect the tanks. Observing what goes on around you is critical to being effective in raiding.
7) Communicate - End game raiding is not for the silent. If you are about to do something like battle rezzing or Ankh, communicate. If you are about to accept the battle rez, communicate. If you are a healer and you get taken off during the valk phase of LK, communicate. If you are a warlock with a portal up and you do not need us to DPS your valk, communicate. The better the communication is, the more effective the raid team is.