This guide is intended for players who are getting ready to start PvE raiding, or have started and are having a difficult time keeping up. Or even intermediate players who just want to improve their game. Rather than giving specific tips on class play, this guide is applies to all classes and all roles. Much of this is relevant for PvP and soloing as well, but that's not my personal area of expertise.  
 
Defining Effective Play
Effective play in a raid is doing whatever is needed to make the raid successful. All three raiding roles have desirable traits players are constantly trying to adopt. For example, these are things like quick reaction speed, strong output, and aggro management. If you try for these traits, and succeed, that's effective. If you try and don't succeed, that's ineffective.

This is different from good play, because good is a subjective word. It can mean a million things to a million people. I've met many players who are far happier to ignore conventions and play however they please. Although this kind of play may not rock the world during raids, who is to say it's bad? You define good play for yourself. Being effective is different. If the effect you want to achieve is taking down a boss as smoothly as possible, then there are specific ways to achieve just that.

That said, here are what I consider to be some key virtues of top raiders.  
 

Critical Thought                         
Critical thinking is being able to rationally look at something with the intent of improving it. Good Warcraft players must possess this skill. Countless players get defensive and put their hands over their ears when they're presented with criticism. Improving play has to be a rational process, not an emotional one. Before somebody tells you that you need to step it up, you should already know that you need to step it up, and be thinking about how.  Understand there is always room for improvement, even in the best players in the world. There is no such thing as good enough. This doesn't mean you should be discontent and pessimistic. Instead you should be excited and optimistic, always looking for the next bit of information on how to improve your play.
 
By the same token, you also need to look critically at advice you receive. Just because somebody has been playing for years does not mean their advice is going to be any good. Don't blindly listen to the first thing someone tells you. Take it under advice, compare and contrast it to other sources, and derive your own conclusion. You'd be shocked at how often friends and guild members have outdated advice they haven't bothered to check up on in the past year. Even guides should be subject to critical thought, though I encourage everyone to read up on their class. Don't accept anything as gospel unless that source has proven to be reliable after time.
 
Research Skills
One of the sad truths about World of Warcraft is that all play styles are not equal. Time and time again, I have seen players solo to endgame, jump into raids, get slaughtered, and then refuse to change how they play. Questing is forgiving. You can do whatever you want and you'll hit endgame, even if it takes half a year. Raids, even the easy WOTLK raids require you start to understand that there are more effective ways to play than others. You can't just make stuff up anymore.

Luckily, there are those who understand the mechanics of the game, and churn out the numbers regular players can use to improve their game. You'll find their information over official class forums, on class-specific websites, or on theorycrafting websites like Elitist Jerks. Their information comes in many forms, but these are the three most important:
 
Talent Builds: We all know this one. Guides written on this will usually list the top few talent builds, or perhaps what you need to know to craft your own.

Rotation: This is the order in which you cast spells to get the job done, although I think spell priority is a better.

Stat Priority: What stats you're looking to stack with regards to your class and spec. Some sites will give you stat weights, which give a point value to each stat. Spell power with worth 1, haste worth 0.67, and so forth.

At any given time, most sites will list the "best" setup for every class and spec. This'll be whatever spec/rotation/stat priority which gives you the highest output of damage or healing with regards to numbers. However, this is not gospel. Just because one spec may put out more raw dps doesn't mean it's right for you. Some talents may need to be tweaked in order to match how you play, and maybe you just have more fun with another setup. Good guides will show you how to make non-standard setups work for you.

The trick here is to understand where you stand, what you're looking to do, and what setup solves those problems for you. Most commonly, players just don't know how to crank up their output for raids and the answer is invariably using the most popular setup. If you're willing to accept you will have to work harder, and likely won't succeed at matching the output of the optimal setups, you can start playing with non-standard stuff that may be more to your tastes. Read the information, understand it and go from there. As always, employ critical thought. If all the guides say the same thing, chances are the information is tried and true, but feel free to examine it more closely. If you're thinking, you're improving.
 

Attitude

This is a constant struggle. Nearly every raid leader wants the same kind of attitude from players, and there's always a few people who refuse to play along. Here are some signs of a good raiding attitude, and how to adopt it.
Pass on loot if it's a sideways upgrade. Not only does this win you instant friends, but it lets somebody who might see the drop as a major upgrade make enormous improvements. The raid improves overall if distribution amongst regular raiders is even.

Don't use the meters as a form of competition. Damage meters are a very useful tool, but they're not to be waved around in the middle of a raid, save them for after. More on this later.

Get excited for raids. Show up early so you can be repaired and with consumables and get summoning on time. It's heart-breaking for a raid leader to hear "Oh, there's a raid? Guess I'll go."

Get your real life in order. If you're a teen, talk to parents and ensure you'll get a few hours to yourself. If you've got a kid, make sure the kid is well taken care of. Nothing earns you enemies faster than a half hour afk without warning.

Keep your cool. Even if one person is driving you nuts, or you're on the fiftieth wipe, don't freak out in the raid and make everybody else as miserable as you. If you can't handle the raid at that time, cut out and avoid drama. Morale is critical to a raid's success.

Be a mentor. If you're doing well, start sharing your information. Give the struggling raiders the tools they need in order to succeed. It's a team, and the team will advance farther if everyone is helping out.

Don't be a know-it-all. If you're brimming with advice and facts, ask people if they mind if you share what you know. Don't be hurt if they say no. Mentoring another player requires some care for that player's ego. Otherwise you come across as an arrogant jerk.

Continued in part two!