Enough time is wasted in LFG as it is. Let's say your last group was terrible, with 3 out of the 4 other players preforming horribly, and now you just sat in queue for half an hour waiting for your queue to pop, then for your next group you get this series of events: Tank canceling his role check, healer afk for the next one, dps afk for the next one, dps afk for the next one, then you finally get in and one guy announces that he can't be bothered to join the group right now because he's busy doing something else, after that you just waited 30 minutes for the checks to pop, then 5 minutes for random anonymous afkers to accept their role checks, and by now you've just had about enough of your fill of dealing with obnoxious people and situations for today. Then you clear all the trash to the first boss, and that guy still hadn't zoned in.
Yeah, maybe if that was a stand-alone event it would be easy to tolerate, but with all the bullshit going down in LFG, people's tolerance for more bullshit being piled on top of them deliberately by other people, to whom they have the ability to respond and thus take out their frustration with the system, is low, and it shouldn't be surprising to see people get kicked for minor breaches in player etiquette.
My advice for LFG is to try to be as nice and helpful as you can towards people, as long as they're at least attempting to preform their role to the best of their ability. If you can't even do that (which you're not doing by deciding that you'd rather finish a quest before joining, when you know that most the others - at least the dps - in that group all also probably just waited just as long as you did to get this thing started) then you deserve whatever you get thrown at you.
If getting deserter for being votekicked right away is not intended, then it should be. It would serve as another incentive not to waste your fellow players' time. I can complete several heroic instances in 10 minutes on any of my tank characters easy (including gate); the only thing that really can slow it down is low dps on bosses. If the OP needed 2 minutes to join a group in Gate of the Setting Sun, then that means his group of "impatient twats" and "jerks" cleared all of the trash before the boss, and stood around at the boss waiting for quite a while before they finally did something about the guy outside the instance preventing them from carrying on... if you're not even going to enter an instance by the time it's been cleared 1/4th the way to the end, then the people kicking you are not "impatient twats"; if anything they're pretty tolerant.
Maybe before you call other people "immature dicks", consider things from their perspective next time.