I run into players doing “speed mode”, which I have yet to figure out. Those always seem to be failures as they take off and I am left trying to fight though everything that they leave behind. They get to some boss and as I cannot get to them, they are a man short. I have had times when the other three players complete the mission and I am stuck at one of the earlier encounters. Most of the time, they get stuck at a boss and decided to kick me for “not doing my job”.
I also run into groups that kick me before I have even loaded into the flashpoint. I don’t usually get an explanation save one time when I was told that I was “under-geared”. That was a story-mode flashpoint, by the way. I look though my gear. It is loaded up with crafted level 50 purple armor pieces and modifiers, leaves me starching my head as to what is acceptable gear and how to get it. To date, I have found nothing that I can actually get that is better save for the daily level 51 armor pieces and that’s only by a few points.
I've had this same experience. A group I was in was in "speed mode" and taking paths with their speeders that required a lot of balancing on ledges to go around mobs. After accidentally falling and causing mobs to attack me, I got booted. Reason given: "bad".
Experiences like this make me not want to group up with random people at all. But I've also done two HM Flashpoints where even though they were in "speed mode", they understood that I was new (I always announce at the beginning that I'm new) and didn't take offense when I would accidentally aggro a mob that they were trying to get around.
After my bad experience, I saw two of my Guildmates on Ilium. So I headed over there and we ran through some of the missions and had a great time. So as everyone else has said: Find a guild that is welcoming of new players and just put people who end up kicking you out of the group (or who are abusive in chat during the Flashpoint) on your ignore list.