Great idea! I like playing 40k, love it infact, its a great way to see friend and an excuse to chat about the most random stuff possible, but sadly there are somethings that can ruin a game i do agree.
So my top 3 would have to be:
1) Taking the game to seriously, yes its a game and so you set out to win (normally), but it should be fun, not something that is win or walk off in a huff and moan.
2) People who constantly have a 'count as' model, I get that sometimes things get left behind, or forge-world stuff (especially) is pricey and they cant afford it straight away, but if you use it every game, why not get the model and paint it or create your own for it.
3) people who don't talk unless its to do with the game, whats wrong with being a little sociable, I'm not asking for life stories, but something please...