First, get out of the tank tree for leveling. The damage is horrendous in tank spec for Guardians, PT's and Assassins can pull it off. Because they have the dps in those trees, and aren't gimped by there resource.
And 2nd don't do pre-50 flashpoints, unless you are ready for pugs like you experienced. Not saying you wont still run into these types of players at 50, but they are few and far between.
but i really think the root of the problem is far too many players came from a mmo where threat generation for a tank was stupid easy and was nerfed to that point because of all of the dps baddies that the game produced. old habits are hard to unlearn. throttling down is unheard of and agro pulling to them is a sign of elitism rather to the rest of us the playing of the unskilled fool. thank god we dont have real time dps meters in this game. what a mess it would make with these dps tools.
Trust me when I say this, I have all 3 tanks class and I tank or have tanked with them all. assassins and powertechs are very much face roll when it comes to threat.
The only tank that has to work for threat is Guardian / Jugg, yes they can tank and keep threat. But they are the only tank class that has to jump through burning hoops.
And yes we have real time dps meters in this game.