One of the few reasons that I rarely do multiplayer any more, unless it's just with people I know.
A buddy of mine complained several times that his session in Farming simulator was ruined because some other people, deliberately placed vehicles into hard to reach areas, forcing him to reset vehicles. Due nature of game, getting vehicle and implement back may be rather lengthy process.
Personally I remember people deliberately jumping in front of vehicle you're driving in BF, only to force you score teamkill and then punish you in addition, to make system kick you, because you "stole" the vehicle they were aimed to use. What is funny is that they left the vehicle instead of using it later, even if it was viable.
Matchmaking that mixed experienced and new players is also quite... "interesting", and I'm not speaking about gear (some games has fixed selection without unlocks), but about skills only. Obviously someone who spent hundreds of hours will be more skilled than someone who just bought the game. Thankfully majority of players just get along, yet there are few, maybe even just one, who will start to gloat how he "dominated" some "newbies". Well, if you like to dominate, why not to play dominoes?
At first I thought I notice these things because I played a lot of "physical" games - be that football (I mean real one, not that uparmoured version of rugby
), hockey, or, I don't know, tabletop games of all kinds, from abovementioned dominoes to ADnD, so people tend to be more... content there, especially in hockey where you could be whacked with goalie stick or smashed on board, outside of usual fistfight, of course.
I don't have much experience with online games of the old, but I have impression people there were all gentlemen-like, in three piece suits. Maybe it was that way, when Internet was not that available as it is now, and people had better manners. Or they always were like it, only because Internet wasn't widely available and few of us could travel a lot, we couldn't see that. 