Let's define properly what a WAAC (Win At All Costs) player really is.
A WAAC is a player whose fragile ego will not allow them to lose a game to anyone. To this end, they will purposefully lie, cheat, and bully their opponent to avoid losing.
If they do lose a game, they will think of every possible excuse for the game to "not count" as a loss. They may even just lie and say they actually won a game that they really lost in order to save face. When they win a game, a WAAC will crow about his victory to anyone who will listen, even if the matchup was obviously unbalanced in their favor. Stoking their ego is the primary reason WAACs play games. They love to grind other players into the dirt because it makes them feel superior.
Some common WAAC behaviors:
1) Using loaded or especially large dice.Though many players (especially new ones) will try a couple of these tricks, true WAACs are actually fairly rare. In the greater Indianapolis area, we only have a single bona-fide WAAC player, who Farmpunk and I call Donkey. Donkey's behavior at the local game stores was so bad that he's been permanently banned from four of the five stores in the city, as well as stores in both Lafayette and Bloomington.
2) Using practiced die rolling techniques.
3) Making "mistakes" in their army lists that give them 100+ more points than their opponent.
4) Slow playing and/or rushing their opponent.
5) Verbally abusing and/or intimidating their opponent (especially when losing).
6) Verbatim use of net-lists (they don't understand how to build their own).
7) "Mis-remembering" or reinterpreting rules in such a way that they give an unfair advantage.
8) Claiming that they've never lost a game (because they only play baby seals).
9) Always using unpainted and/or unassembled models (to confuse their opponent).
10) Purposefully sloppy measuring that always adds 1-2" to their moves.
11) Not adding up their kill points properly at the end of the game.
12) Unsophisticated tactics (they like to keep it simple and brutal).
13) Threatening or yelling at the other player when they get caught cheating.
The only other place I've seen true WAACs is at the 'Ard Boyz semi-finals. Where they come out of the woodwork due to the large prizes, random missions, and flawed battlepoint system that encourages winning through seal-beatings.
WAAC =/= Competitive
The antithesis of the WAAC is the competitive gamer. Competitive players love to be challenged. For them, a win feels empty unless it was against an opponent they respect. While a loss to a good opponent is considered a challenge to be overcome.
Some common competitive behaviors:
1) Avoiding new players or those with poor win records, because they're not challenging.Competitive players can seem like WAACs to new or less skilled players. But this is because they're not playing the same game. A hobby-centric player who only fights 8-12 casual battles a year with friends simply doesn't have the same understanding of the game that a competitive player who fights 50 - 100 battles a year does.
2) Seeking out the best player in the room to see how they match up in a game.
3) Keeping detailed win/loss records in a spreadsheet to track their improvement.
4) Taking less optimized lists when facing newbies to increase the challenge of the matchup.
5) Giving advice to newbies they play mid-game, in order to improve their skills.
6) "De-briefing" an opponent after the game to try and figure out how they could improve.
7) Helping a new player with list building.
8) Absolute hatred of WAAC players.
For one, competitive players know their rules. Two competitive players will almost never have to look at their rulebook or codex during a game. Top level players can craft entire army lists in their heads, as they've memorized all the common options and costs in the army book they use. They also know their enemy's army fairly well. Either though playing them over and over, or because they've bought and read the codex. Even if they don't play the army themselves.
Second, competitive players know how to build an effective list. Whereas a new player will craft a list based on what looks cool to them, an experienced player has tested every option. So he only uses the units and options that both work for his strategy and which are efficient for their points cost.
And speaking of strategy, competitive players have one. Whereas less experienced players will simply line up on either side of a table and pretty much just shoot at each other, competitive players know how to move. They have a plan for what they'll do in each of the first 3 turns and a pretty good idea of what they'll do after that. The best ones can read an opponent and know exactly what they're likely to do as well. They'll also know by your body language exactly when you've given up. Even before you do.
The result of all this is comparable to what would happen if Michael Jordan were to play a pick-up game of basketball with someone who plays once a week at their church. A newbie/hobby 40k player will be dazed as his units are hit with an unbelievable amount of force in just the right place by an army that seems to be way too large for it's points level.
Nothing he does in response seems to make any difference, as his best units get beaten down in a single turn or less. He may also find that the actual rules of the game are stricter about things like assault movement than he's used to playing. Or he may not have realized that true line of sight means that a unit in cover can shoot at another unit without giving it cover in return.
And so the dazed newbie will often throw out the term "WAAC" or "Cheesy" to describe the player that just thumped them. But the real issue is that a low level player didn't enjoy his game against a high level player because his skills were not up to the challenge.
But the competitive gamer hears "You're a cheater!". When all he's guilty of is being a better player.
The real answer is just to be honest about the kind of game you want to play before you throw down. That way nobody's expectations get disappointed.