Bashing other games is a big one. Certainly guilty of it.
I think we do it as we like to think the path we have chosen is the better one, and the path we left behind is suddenly very wrong, so we have to bring unbelievers to the light. "Repent of your heathen D6 ways!". There is some tribal stuff going on too.
A lot of Infinity players I know enjoy multiple systems, and in helping to build my own community i've had to stop the bashing. I easily forgot what fun I did have with 40k before it no longer worked for me.
Showcasing what the game does is definitely the way forward. And respecting the gamers of other systems. A buddy sent me a text the other day saying he was "playing a real game" (40k). Instead of replying with a tirade I just said I was happy he was getting game time in no matter the dice shape.
I also don't frequent 40k/newbie gamers' fora, and that's where the quitters seem to head (if anywhere), as we rarely have them in the Infinity ones.