My Main is a Vangaurd Shield Spec, and I am a tank.
I have never had a problem tanking in heroics or flashpoints, why? Because regardless of who the group leader is, the tank controls the ebb and flow of the mission.
If you are the tank, then take charge of the group, its as simple as that. Healers keep tanks alive so that the pew pew's (or, in this game froom froom's) can go ahead and slice, dice and shoot holes in the baddies.
If they dont want you controlling the speed of the mission based upon Healer cooldowns, and your own taunt/mitigation cooldowns. Then leave the group.
Second in charge should be the Healer...
THEN the CC/support types...
THEN the DPS'rs...
DPS players should never be lead on anything, nothing against them, I like playing DPS toons as well. But they tend to make things harder on everyone else.
Just my two cents. If the peeps you are grouped with do not like that, oh well, leave, and let them find someone else.
Rolling a tank doesn't give you the right to be your own little Attila the Hun. Good groups work together, converse and strategize, and try to get the most out of everyone's abilities. Same goes for healers, CC, DPS, ANYONE. No one is better than someone else or more important. I've seen healers who refuse to heal DPS, and then wipe over and over again because they don't realize that you can't win some fights without an appropriate amount of DPS. Everyone should be doing their job, and if there is a problem handle it like adults.
As one fella above mentioned, I play Jedi Shadow (mirror to Sith Assassin) and I also DPS the normal mobs in pack pulls. I do this for many reasons, the first being that it is easy to do. I can usually down the normals quickly and assist on the elite(s) once finished. I take almost no damage doing this, and I even have tanking cooldowns I can pop if needed. I sometimes even switch to Combat and put Guard on the healer to be extra cautious on some fights (I'm looking at you Council fight in Mandalorian Raiders).
Many players have this old school mentality that the Tank and Healer are the center of the world, and that everyone else is just there as chaffe. This isn't true anymore, and it shouldn't be true in good game design. Every member of the group should have a role, and there should be some encounters where that role is flexible or dynamic.
This mentality that the Tank is "in charge" needs to go. Get over yourself. I'm not here to follow your target of target and mash the same rotation until everything dies in the order you determine (erroneously or not).
Don't make me remind you how often it is the Neanderthal tank types who charge into every pull without letting the CC classes utilizes their abilties (Sap much? Oh you already aggro'ed, nvm).
You want to complain or get on a soapbox about good playstyle? Teach people to fill their role and to do it with some sense of logic. Until you are maximizing every advantage and minimizing every risk on EACH and EVERY pull, you have no room to talk.
SWTOR is a revolutionary game in this aspect. The lines have been blurred, each class has multiple roles and isn't pigeon holed to just one playstyle. Has it ever occurred to anyone that perhaps the old ways aren't applicable here? That perhaps these encounters are designed to be different?