Why is it always Sents/Maras?

Pretty much the same from what I've seen in over three years of tanking and healing. Most Sents and Mara's are pretty good. It's the few that give the class(s) a bad name.

Regarding kill order, most dps are clueless, that's probably never going to change.

You don't really think that would make a difference, do you? People play double-sabers because they think it looks cool. They do those other things because thinking you (and I don't mean you, personally) are good enough to play outside the mechanics of your character's role is a pretty common among "cool" MMO players who vastly over-estimate their gaming abilities..

"Is it something about the class itself that attracts people who refuse to listen to reason?"

Yes, yes it is. Two light sabers.

Why did the sentinel force leap into the hallway?

To get to the other side.

As you can see by the dates on these links, this is not a new observation or complaint.

Sentinel/marauder complaints are nearly as old as the game itself.

Of course it's not all sentinels and marauders, but it does seem disproportionate.

There is when you're playing Sin tank.

*Force Shroud / Force Cloaks / Force Speed away"
*Sit with popcorn while said Mara dies*
"Well I did ask nicely"

Oh hello there, Satan.
XD

Because in the hands of a good player Sents/Marauders can tank.

On the other hand, a good Sent/Mara is a thing of beauty that should be adored, protected, coddled, and applied liberally to all enemies. There's nothing more disheartening than being a good DPS that gets treated badly without being given a chance to prove otherwise.

Here's a tip: If your Sent/Mara uses phrases like, "What's the plan?" or "Who's the tank?" or "Does everyone know this fight?" then there is a strong possibility that this person knows how to play a DPS properly and you should a) answer their questions and b) heal them. Please don't just assume that everyone who plays the class is horrible and should be ignored.

I follow this strictly. I won't peel or heal if a DPS pulls.

Root cause: faux Sents/Maras with poor impulse control, played by real people with actual impulse control issues, who fantasize that they are a "real" Jedi in a far away galaxy. That and they most likely don't give a crap about the rest of the group. Most of these folks are probably ex-Huntards from WoW... which gave real Hunters a bad name on an almost perpetual basis.