Heretic here again to talk to you all about etiquette and a professional attitude in our beloved hobby. If you haven't read this article, I suggest you take some time to do so. Goatboy is generally a pretty decent commentator on 40k and this article does not disappoint. His points about dice rolls and transparency in all aspects of gaming are good, but there is more to being a good player, in my opinion than that. Don't get me wrong, if you follow what Goatboy says and common courtesy, you will generally be a good person to play against.
What I see as professionalism isn't necessarily taking the game with the seriousness that you would your job, but having an attitude conducive to a good game. A player's conduct is under their control and only their control, so it is the core of what I view as an attitude conducive to a good game. In some areas, leniency is very helpful to conducting an enjoyable game. For instance if you and your opponent are not sure whether a vehicle or monstrous creature is half or more obscured for getting a cover save, either remember the provision in the Big Rule Book (BRB) for a 5+ save in case of disputes or simply give your opponent the benefit of the doubt. I've found that a lot of those "iffy" cover saves happen to fail, but the important part is producing a friendly atmosphere with your opponent. If you give them the benefit of one iffy call, they should reciprocate later, making what could be a hotly contested point, slowing the game and raising tempers, into a minor point where friendship was worth more than the game, as it should be.
When it comes to rules disputes, I will admit my tendency to be a rules lawyering ass. The rules are the foundation of the game and give it structure. Much like the lines on a football field, the rules say where one can act and where they cannot. The codices and BRB define these limitations in, admittedly sometimes obscure, language whose meaning is generally clear. Houserules are perfectly fine, if they are agreed to by both players before hand. Only bringing up houserules midway through the game to explain your action can leave your opponent feeling as if you are cheating. To avoid this perception I try to know what my codex says, but we can all mess up with our memories from time to time. Remember your own falability and be willing to look up the answers to any question your opponent has. Don't act like this inconveniences you, even if it does. Literacy in an army is always helpful as a player.
When an opponent makes a choice you think is an error, for instance firing a Strength 7 weapon at a Land Raider (AV 14), try to politely ask them if they are sure. We all make mistakes and we don't always see them. Discussing with your opponent why a choice may or may not be a good idea will slow down your game, but it will get both of you to think. If your opponent realizes their error and wants to take it back, in a casual game, I say let them, within reason. This may put you in a worse tactical situation, but it will put you in a better interpersonal situation. Your opponent should realize that you would have been fully within your rights to force them to adhere to their decision, but chose not to. In a tournament, I would not point out errors to my opponent until after the battle, but even then it is an opportunity to discuss your way of thinking, not for gloating.
The classical Greeks believed in a principle they called "Agon," from which we derive the word agony. For them it held a very different meaning; Agon was not suffering, but the struggle inherent in sporting and contests, generally at religious festivals. I love to play this game for the same reason I loved to wrestle: there is a definite sense of Agon on the field. One person on each side pits their mental faculties and luck against another, trying to see who is the better on that day. Winning is a good feeling, but I've never really liked the feeling of blasting an opponent off the table nearly as much as counting up the points at the end and seeing either a draw or a minuscule victory for either side. Neither side towers above the other, but each gave as good as they got and gave their best fight, even if it was not enough to win.
The willingness to struggle against an opponent, to help them and give them the benefit of the doubt are all signs of respect for your opponent and yourself. This is just a game we all play, but that does not mean that giving your best performance in your conduct, your tactical decisions and being willing to give some leeway for your opponent say that you respect them as an opponent and treat both yourself and your opponent as adults by acting in a professional manner.