I ain't talking about the mod. >.>
"Rushing" very fast through low level missions! Alerts mainly, Invasions also this happens a lot. Maxed out players join low level games, and the low level players complain that everyone is just "pressing 4 every room" and "going too fast" (just some things I heard someone say in region chat). They can't enjoy the game!
But at the same time, isn't it selfish to ask to the high level players to "slow down" to play at your pace? It's their time too - they could be busy people...
Really don't know what to think. At the same time I think the Invasion system could be designed a little better. Forcing someone to do the same mission 3-5 times in a row... of course after a while they'll get sick of it and just want to blast through it every time. Then there's the random matchmaking. You cram massively different levels of players together there will be friction no matter what you do.
And at the end of the day maybe lower level players like getting carried through Invasions with little effort on their part? Who knows what could be considered "bad manners" (or is there even such a thing in this game).
Personally... I'm a little guilty, but I use Alerts/Invasions as an opportunity to level up my unranked guns. Nobody seems to notice >_>