The hunter took a couple of steps backwards, away from the people meleeing the mob, but got into the aggro range of the next group of mobs, wiping the raid. The priest had a flaky internet connection, getting disconnected several times during fights, also wiping the raid. Next raid the main tank warrior didn't show up, there was no replacement available, and the raid was cancelled after a frustrating wait. One raid everybody relied on the paladin to heal, but he had respec'd to retribution for PvP and hadn't told anybody, causing lots of wipes. The mage is always starting dps too early, before it is called. The warlock suddenly had to leave in the middle of the dungeon, for some real life nonsense, and it took half an hour to replace him. And don't get me started on the new guy, the shaman, who doesn't know the raid encounters and keeps making mistakes, while getting most of the epics that nobody else wants any more.
Sounds familiar? Fact is that whether it is 5-man groups or raids, somebody making a mistake or having an accident can easily wipe the whole group. It doesn't matter how good you play, if the guy next to you messes up, you'll wipe too. The harder the encounter, the less room there is for error, and the smaller an error from one player can kill everybody else. Repeatedly. Obviously that is quite frustrating, especially if it wasn't you who made the mistake. And if enough of those incidents happen, it doesn't exactly evoke love towards your fellow players. Some groups or raids simply end in failure, or even don't get started at all, and you leave you cursing and fuming about the others.
That doesn't remain without consequences. My headline of "hating the people you play with" is an exaggeration. But you don't have to look far on various official game or guild forums before you stumble upon posts where one player disses another player for one of the things I mentioned in the first paragraph: Playing badly, getting disconnected, not turning up, making a "wrong" choice of talents, et cetera, et cetera. And many guilds have formal or informal rules that kick people out, or at least not give them raiding spots, for various "crimes". I've read one guild web site where going on holiday for 3 weeks (as I will in 10 days) is enough for getting your kicked out of the guild, even if you announce it beforehand. Most guilds are far more moderate, but if you have a history of getting the others wiped, don't count on getting invited to the next raid. Even if the problem was related to your internet connection or real life happenings, which aren't really your fault.
But the most serious consequence of other people being able to ruin your fun in a group is that it often results in people preferring to play solo. You would think the main attraction of a massively multiplayer game would be to play with other people, but at any given moment in World of Warcraft there are far more people not grouped than in a group. Playing solo is popular, because it minimizes negative interference from other players. The cost of that is that it also minimizes positive interference from other players. Forced grouping, like in Everquest, although it did foster social cohesion, is now so unpopular that you simply can't sell a game any more that does it. But the degree of friendship you feel towards your fellow players has diminished, leading to phenomena like the Kleenex guild, disposable at any moment.
I'm not quite sure how it can be done, but I think that game design of new MMORPGs should be tuned in a way as to make players not despise each other so much, but see them as allies. Grouping shouldn't be forced, but I think there is room to make it more advantageous, giving better xp bonuses for grouping. And in designing encounters for groups you need to carefully balance the challenge and rewards of everybody playing well with the likelihood of one error wiping the group, and the resulting frustration for all other players in the group. Can they just try again, or is there a respawn throwing them way back in their progress if they wipe? Multiplayer games shouldn't teach us to hate the people we play with. Not just because that "isn't nice", but for the simple business reason that people pay longer for games where their friends are.