Quite a bit of everything that you mentioned.
I didn't get angry about it, because this was my first real participation and I can just ignore those types ongoing. Another tactic that I found a bit peculiar was a predilection on the part of some gamers to bottle themselves up in the final stage at the entry point once the flag had been planted? Sometimes that helped whittle down waves of enemies coming to you, but it also dramatically allowed some area effect attacks to wipeout our party on a couple of occasions.
I will agree with the speed-running being annoying. It may be more efficient in the long-run, but I much prefer to have the adventure of treasure room hunting. I usually just go with the flow of what the others in the lobby want to do and if I get pulled through a speed run, I'll just find another lobby afterwards. Different strokes for different folks.
Not reviving is not cool, but that'll always be a mixed bag. And sometimes, you can miss seeing someone in the heat of things. I missed someone yesterday because they were right near some bushes and I was so focused on the Druffalo... I felt bad afterwards because they were obviously angry at me (I had been the closest to them and didn't revive them) and kept shooting me with arrows right afterwards, and I rightfully deserved it. But I hadn't ignored them on purpose, I just was flat-out too zoned in on the thing I was shooting. So, may not always be an intentional snub.
As far as stuff getting killed really fast, since you just started I assume you don't have a lot of good weapons unlocked yet. It'll take time, but as your weapons get better, you'll be able to kill faster. It's sometimes hard for others to reign it in, especially if they're rushing through a new set of promotions or whatnot.
That Wave 5 tactic is one I do see a lot. Sometimes, it works out pretty okay. Other times - like when the room gets invaded by the Demon Commander or Red Templar Commander - it turns into a killing field. It's a mixed bag whether you get a group that's savvy enough to realize this and vacate.