I read an article on Cracked.com once (no really, bear with me), in which the author explained that road rage is caused by us perceiving other drivers as disproportionally rude due to a lack of visible body language during their maneuvers. Now, I don't know how much of that is actually based on fact, but I do have to say that the basic idea that a lack of subtle signals can lead to a lot of miscommunication between people sounds very logical to me. I also can't help thinking that this is what must be happening in a lot of pugs gone bad, especially as I've observed a lot of surprising reactions to things that I've said and done in my own random groups over time. Keep in mind that I'm someone who tries to be mindful of these things, so how much worse must it be when people don't even think about it?
For example, I was healing a normal Drak'tharon Keep run the other night. The tank's health hardly moved while we made our way through the first hallway, and I settled in for a fairly relaxed run. Then we came to Trollgore's room, the tank pulled, I saw the dps taking some damage from the abominations' aura, healed them as well... and suddenly the tank's health plummeted like a rock, as apparently we had got the entire room or something close to it. I wasn't fast enough to get him back up and we wiped. As we ran back the tank said that I should just spam him, and I responded with "sorry, I was trying to heal the dps as well". This was my attempt to summarise in a nutshell what I just spent a whole paragraph explaining on here: that there was some AoE going around, I hadn't anticipated such a damage spike on him etc.
However, I quickly realised that these words could also be interpreted in a completely different way, for example: "It's not my fault, I had to split my healing because you weren't soaking up all the damage like a good tank should be doing." It's not what I meant at all, but it soon became clear that this or something similar was what the tank had actually read. We didn't fight about it and just moved on, but it sure made me think. At first it seemed silly to me that the tank had interpreted my words that way, but after some thinking I had to admit that I might have done the same in his position. Who hasn't been burned by a pugger who criticised them mercilessly for the smallest of things? Who isn't prepared for a certain extent of bad play and mean behaviour when grouping up with strangers? Assuming the worst as soon as someone speaks is almost natural, and so is getting defensive.
If we want to do everything we can to be on good terms with our fellow puggers, we have to try to communicate more clearly, instead of trying to make the run a few precious seconds shorter by only typing a couple of words. We have to think about how our words and actions could be interpreted from a different point of view and try to make our intent as clear as possible, more so than we would usually do in real life that is. Add smilies! I know many people look down on them, but a simple ":)" at the end of a sentence can add so much to your message. The people in your group can't hear or see you, and continuing with the conclusion from above that people will always assume the worst, they'll probably think that you're grouchy and aggressive. Adding a smiley can instantly defuse that tension, showing everyone that you're friendly and not out to ninja their loot or insult their play style.
I have seen how much of a difference these little things can make as well. For example I keep getting hugely positive reactions to something as simple as announcing that I'm about to make an off-spec roll. I always wait to see if someone needs the item for their main spec first and only roll need if nobody else did, so some people might say that my intent should be obvious anyway and that talking about it is just overexplaining. But it's not, and the fact that I waited to see other people's rolls first is easily overlooked. By stating explicitly that I only want the item for my off-spec I'm putting possible main spec competitors at ease. It says, "I respect giving main specs priority over off-specs, don't worry. I won't be that dps death knight who ninjas your tanking trinket from the last boss." Those who just roll need on everything won't care, but those who still care about main vs. off-specs will, and they'll immediately have a much more positive attitude towards you than before.
Hell, even the little "hi" at the beginning of a dungeon helps. I have to admit, I had given up on it for a while some time ago because it felt pointless. Nobody talked for the rest of the dungeon anyway. But then I read a discussion about the quality of pugs somewhere (I forgot where) and one of the people defending them as not all that bad said something like, "I always make sure to say hello at the start of the dungeon and most of my groups are pretty polite. Maybe that makes all the difference?" It seemed silly, but I did pay more attention to it afterwards, and I actually always feel better when the majority of the group says hello at the instance. It's a last concession to the fact that we're still playing with other people instead of random NPCs, and it always gives me hope that the other players will be more likely to remember that there is a real person behind the avatar if anything goes wrong and tempts them to lash out.
In conclusion, don't be that guy who never says anything. In theory there is such a thing as talking too much in an instance, but it's been a long time since I've seen anyone who came even close to deserving such a description. Talking in a friendly manner is so valuable these days, as it reminds everyone that you're a person and want to deal with them on the same level. I'll admit that in a run where everything goes smoothly it doesn't matter as much, but if anything goes wrong at all, I know that I'm not happy when all the other avatars just stare back at me silently, pretending that they didn't see the question I just asked in chat. And I don't want to give others that kind of treatment either. I think it would make a lot of runs a lot better if more people kept that in mind.