Another cause for this was how especially Paladins when geared correctly had virtually unlimited mana, and would nearly nonstop spam their biggest heals on the tank. Which indeed meant the only way to threaten a tank's life was to do nearly fatal damage within a GCD. Considering that this sort of scenario isn't particularly likely with gear independent regen rates, I doubt (and hope) we'll never get to that point in ToR.

Once you complete the questline on Tython, you get sent to Coruscant through Carrick station. On arriving at the station, you will receive a quest directing you the advanced trainer and allowing you to choose either Shadow or Sage. (Thereby making the talent trees and skills available for the advanced class of choice)

That was actually something I was wondering about earlier today looking at the talent trees. Purely from a DPS point of view, Force in Balance seems quite nice as it does more damage than Slow Time and is slightly cheaper in terms of force, and also provides a fairly small heal. On the other hand Slow Time gives the debuff, affects more targets and has a lower cooldown from what I can see. (And I'd guess in high end operations the debuff is going to be the dealbreaker in itself) It's also cheaper in terms of talent investment (1 vs 3 points), though to get to 31 points you end up with a couple of highly optional talent points spent in Kinetic Combat. Either way, I ended up wondering which ability will actually provide more threat per target per use. Force in Balance obviously has the higher damage, but Slow Time according to the tooltip provides high threat. Anybody got any idea about the threat multiplier on it? (Assuming it's going to be a multiplier anyways and not a static threat bonus, given how many scaling issues that gave in other MMOs)

You end up spending a lot of talent points in talents which don't help you all that much. Lots of buffs to Shadow Strike , stealth play and burst potential. Might be nice for PvP, but for tanking it's utterly pointless since you won't get any tangible benefits from those talents after the first couple of seconds at best, if not at all. (tanking shadows do not use Shadow Strike since when tanking it's not possible to get behind the target consistently) You'd be much better off taking Balance as a secondary spec, since the lower talents there actually offer nice benefits to add to your tanking talents in Kinetic Combat. (Jedi Resistance, Psychokinesis and Upheaval work well with the skills shadow tanks use.)

Regarding viability of Shadow vs Guardian: It's true that Guardians wear heavy armor and Shadows wear light. However, Early on Shadows gain a skill (I believe Technique) which massively enhances their armor and should put it pretty much on par with Guardians. (Think of it as a tanking stance, kind of like Stances/Presences work for Warriors and DKs in WoW) Overall Bioware has been quite open about their intentions that all advanced classes which spec purely for a certain role (ie go deep into the tree appropriate to that role and pick a sensible combination of talents) to be equally viable at the given role. So for example the idea would be that a DPS specced Shadow, Sentinel, Guardian and Sage to do equally viable DPS in a different way (But with the Sentinel having more options to spec DPS, so likely having more variety or flexibility). Similarly Guardians and Shadows should be equally viable, but play differently. Overall at least on paper the way Bioware has indicated things, it should purely be a choice based on which style, aesthetics, lightsaber, etc. appeals more to you. Given other MMO's there will never be perfect balance, but the intention is that things are balanced between classes. (And I don't think there's enough known about classes, scaling and what not at 50 to figure out if one tank is more viable than the others) Since to me the Shadow style and concept appeals more, for me the choice ended up with Shadow. As another note, during beta quite often the concept popped up that Shadows/Assassins would be avoidance tanks (so mitigate less, but avoid more, which generally leads to more bursty and random dmg intake, something healers often hate). This is also a misconception which I think got formed due to a comment by one of the devs, and might have gotten strengthened by the somewhat acrobatic nature of the Shadow. Reports from beta indicated however that overall mitigation and avoidance for the tank classes is roughly in the same ballpark and not radically different.