L4D(2) community is BY FAR the worst of any valve game. Even worse than Dota2's.
It is, but only slightly, and I think it naturally makes sense that it would be. Both Dota 2 and L4D2 are team games, but L4D2 is a much more intimate team game than DOTA 2. DOTA 2 is obviously a way more complex game, but you can also, to a certain extent, kind of just spend your time farming in DOTA on your own for long periods and it has a looser structure (higher levels of play, obviously, will demand more communication, but low level pub games not so much). Whereas, in L4D2 Versus, you absolutely have to be next to and constantly working/communicating with your team the entire time. Especially as infected. The infected are such that they are entirely useless on their own and their attacks only work in combination. So given that the game demands players be working directly together at all times, I think that might explain why the community might be more hostile (plus, L4D2 doesn't have any of DOTA's community features like the ability to rate people as "friendly" and such.....which I actually think would lighten the mood in L4D2 somewhat, but I doubt they will add them at this point).
That said, if people are serious about playing L4D2 there's alot they can do to mitigate negative experiences.
The mistake most players make is not playing campaign before they jump into versus. I did nothing but play campaign before bothering with versuses. I got the "beat all the maps on expert" and "beat a map on expert realism" achievements (which took about 60 hours), so I was familiar with the basic strategies employed on each map before moving to versus. You will still get kicked in the early going as you learn the various team strategies to employ in each map (especially in regards to when/where to employ death charges, which is a huge thing in VS), but you pick this up pretty quickly. After about 40 more hours or so of play (i.e. 100 total), I settled into a groove where I was able to have fun with it more often than not. I now have 900+ hours, almost all spent playing pub versus games, and bad experiences are pretty rare for me. The community definitely can be kick happy, but it's easy to avoid that if you know what you're doing.
Although, I also think you have to thrive on conflict resolution to enjoy the game, to a certain degree. Part of the appeal of the game to me has always been the way it forces complete strangers to work together. I think L4D does that more directly and simply than most other MP games I've played. DOTA2 does the same, but its rules are so complex that not everyone knows what the goals should be, whereas L4D2 distills this down to a much simpler form (although I think the strategies employed can stil be complex). So I'm able to enjoy the game as much when my team is working beautifully together as when there might be a weak link who we have to carry or the team is potentially falling apart. I can't really think of many other MP games whose core gameplay mechanics better realize and embody the game's overriding theme--as the whole point of zombie movies has always been strangers being forced to work together to survive. That's what's always attracted me to pubs and keeps me interested in the game even when I'm losing---the fun of having to try to work with people you just met 2 seconds ago and don't know from Adam.