A few easy tips for steering clear of the most common ways to ruin a game in short order.
Talk to your opponent.
This bit isn’t really part of a tournament game, but before you get started in a pickup game at your local store or club it really pays to talk things out with your prospective opponent. What kind of army are they playing? What kind of game do they want? What missions? Some players might want to practice for a tournament, while others might be looking to try out a wacky scenario they found- and if those two players have a match without discussing their expectations, they are both going to have a bad time. There’s no right or wrong way to play the game, but you do need to discuss what you’re looking to get out of it because the other guy may be looking for something else.
When you walk up to the table, smile and say hello to your opponent; shake their hand, introduce yourself, and go through all that rigmarole that’s expected. The reason it’s expected is that politeness is the grease of social interaction- it keeps all the parts moving smoothly and easily. In any setting where you don’t know the player personally (i.e. most tournaments and pickup games) both of you are starting with blank slates, for better or for worse; starting things off on a positive note will go a long ways towards making a good impression on the other player and thus making for a good game.
Now, I realize not everyone in this hobby is an outgoing extrovert who thrives on attention and conversation, but that’s not really the point. I’m something of a misanthrope myself and one of my regular tournament traveling companions is a diagnosed autistic individual, but we both still manage to smile and be sufficiently perky for normal purposes- and in light of that, no one else really has a better excuse not to. You don’t have to be a smarmy gladhandler, you just have to be nice enough that people don’t actively resent your presence.
Go over terrain and armies
Different groups play terrain very differently- some make hills difficult terrain to get up the sides, or only on certain sides of them. Some play all ruins as having specific distances for each level and some measure it all inch-by-inch. Some count all ruins as having a “footprint” to match their linear dimensions, and some insist that only ruins with bases use those rules. Especially if you’re playing at a regular club or shop event, there may be certain “agreed” rules that all of the local players use for specific terrain pieces. No matter how you do it, I guarantee you that someone, somewhere else plays it differently and discovering this fact in the middle of a game can be a really great way to ruin the experience for both of you- so save yourself some trouble and go over things before the game starts. What cover save does everything count as? What is impassible, and what is difficult? Do any terrain pieces qualify as special types, like forests or ruins? What are the boundaries of different pieces? What can and can’t be moved through or over? Terrain makes the game leagues more fun, but only if you can interact with it- and you’ll want to ensure that both you and your opponent have the same assumptions about everything.
On a similar token, go over your respective armies before the game gets underway. This is pretty second nature for tournament players, but it’s just as important for casual players- no one wants a nasty surprise to crop up during the game and ruin things. The game is more complicated than it’s ever been- make sure the other guy knows what your different detachments do and what units they contain, which psychic powers and warlord trait you rolled, and all of the other little details about special rules your army possess and whatnot. Some people are familiar with “enemy” factions, others less so- it’s always good to check and make sure. If the other person doesn’t know your army, give them a quick (and honest) overview- where the big guns are, what the major tricks are, etc.
Be clear about what you’re doing and why.
One of the most frustrating things during a game can be the “mystery dice” phenomena where your opponent rolls a sequence of dice, calling out results (“Two hits, two wounds, three saved, roll six dice for me.”) without explaining what is going on. When you roll dice, it is good form to announce what’s going on (in basic terms) to your opponent- especially so when you are manipulating the results of the dice (i.e. through a reroll) or have modifiers to a normal roll (i.e. Stealth, Markerlights, or other bonuses.) If you just sort of reroll things whenever you want without saying anything about why, your opponent is going to feel like they are being cheated even if what you’re doing is 100% legit. I’ll often announce the whole sequence of an event, such as “My Crisis suits are shooting plasma at your vehicle- needing threes to hit you thanks to my Markerlight counter, so that’s five hits and then fours to glance with a reroll from Tank Hunter, so a total of three pens and one glance- take your cover saves.”
As we’ve already said, 40K is a game with a lot of rules. A lot of rules. We all forget things sometimes, be it moving a unit, resolving a combat, or maybe even an entire psychic phase. Letting your opponent take back something they did, change a decision, or retroactively take an action they missed goes a long way towards convincing them that you aren’t a hideous mockery of a human being. Of course this isn’t a privilege to be abused infinitely- if they keep forgetting things time after time, you have to draw the line somewhere, but it’s usually pretty reasonable to let them have a mulligan or two unless they have been exceptionally awful to you already. Remember that this goes both ways- if you let them go back to something, it can be expected that they will give you the same permission, so it needn’t even be an entirely altruistic act.
Take the time to notice the opponent’s army.
We play this game because we love our armies; we put hundreds (or thousands!) or hours into them building, converting, cleaning, painting, and optimizing them and we drive or fly large distances to gather with other folks and show off all our work. We like it when people notice what we’ve done and appreciate it, and this is wholly reciprocal- our opponents like it when we do the same for them. While you’re playing the game (or getting ready to play), take a moment to observe your opponent’s army. Does it have a really clean paint scheme? Did they do some interesting conversions? Do they have a really thematic presentation for their army list? Are they using an unusual unit or upgrade that isn’t often seen? Did they achieve a really unique effect with their basing or paint? Ask them about it- most players are more than happy to pontificate about their army for exactly the same reason we each love telling other people about our own army- it’s just human nature. And, all pandering aside, many players really will have something interesting to tell you or teach you about the hobby- you might find a really good idea for a conversion or paint scheme later on or a technique you hadn’t even considered if you take the time to ask.
Don’t get upset over bad dice.
40K is a game of dice; that means, by definition, it is a game of randomness. Human beings are incredibly bad at understanding how statistics and randomness actually work- we see a roll with four sixes out of ten dice and rave about how incredibly unlikely that is while completely forgetting that we have rolled dice thousands of times in a single game and it should come as no surprise at all that outliers will pop up occasionally- in fact, it would be strange if they didn’t. So when the dice go bad for you, just accept it; go through your little rituals and superstitions if you must, but realize that it’s only temporary and it could just as easily turn around at any moment and happen to your opponent as well or instead. No one will blame you for being frustrated at bad turns, but that doesn’t excuse the childish temper tantrums and complaining that are so common to the hobby.
Corollary: it is incredibly bad form to rejoice at your opponent’s bad dice. C’mon now, is this not obvious? As shitty as you feel when you can’t roll a three to save your HQ’s life, your opponent will feel just as shitty when it happens to them. Don’t rub it in- commiserate with them to the extent it feels appropriate and keep on with the game. If you pump the fist when your opponent fails an important roll due to awful luck, you’re kinda being a dickbag. (Of course, there are times when the sheer improbability of an event baffles, astounds, or even impresses both players- in these cases it is fine to share your opponent’s reaction, but if he’s unhappy about it then you probably shouldn’t be celebrating.)
If you’re not enjoying the game, end it.
Sometimes it happens that a game doesn’t go well- someone makes a mistake that tips the balance of things too far, or your luck is bad and you’re really not feeling it, or there have been some bad interactions between the two of you. Sometimes games just don’t go well; it happens. Don’t feel ashamed about admitting that you’re not enjoying the game and you’d like to call it- it doesn’t have to be an aspersion on you or on your opponent, but just a simple fact of how things sometimes are. Now, it also can happen that there are rough patches in games that you can get through with just a bit of determination, so it doesn’t really pay to just give up on things immediately as soon as anything goes wrong, but if you genuinely are not having any fun and don’t see any chance that you will down the road with the game, by all means stop it. This is a hobby, not a job- you don’t owe anyone anything other than basic respect when it comes to playing with toy soldiers.