http://www.swtor.com/community/showthread.php?t=156809This has been discussed some in Taugrim's thread, but I think it deserves its own post to better spread awareness.
There are no combat logs or parsers, so instead I tested all of this with good old fashioned tedium. I spent quite a long time yesterday on Dromund Kaas letting my very dedicated girlfriend's buck naked Operative stab me in the back while Mako and Dr Loken had a fist fight.
I'll start with the good news:
Defense and Shields in this game are omni-directional. Unlike some other games where you could only block or dodge in a frontal arc, a qualifying attack from any direction may be shielded or avoided (Dodged, in the case of Bounty Hunters).
Additionally Shielding works while stunned so long as it is a qualifying attack.
Now for the bad news. You're not going to like it.
Above I highlighted the significant phrase "qualifying attack." While the in game tooltips for defense and shielding seem to indicate that they will protect against any ranged or melee attack (which logic dictates must be most if not all of them), this is nowhere near the case currently.
As it stands, both Defense and Shielding only protect against an attack which does "Weapon Damage." I haven't exhaustively tested, but so far these all seem consistent with their tooltips.
What does this mean? Well, take a moment to look at your own bars. Imagine you are facing another Bounty Hunter, and take a quick moment to appraise the skills to see what your Defense and Shield will actually protect against.
I'll save you some counting. Three. Rapid Shots, Unload, and Rail Shot.
Rocket Punch deals Kinetic Damage. It bypasses your Defense and Shields entirely. Still mitigated by Armor.
Flame Burst deals Elemental Damage. It bypasses your Defense, Shields, and Armor entirely.
I'm sure you see a pattern emerging here.
This sounds pretty bad. In practical application a vast majority of a rival Powertech's damage is going to bypass Defense and Shields, and another good chunk doesn't care about your armor either.
How deep does this rabbit hole go exactly?
Operatives only have three attacks which are mitigated/avoided by Shields and Defense. Rifle Shots, Overload Shot, and Carbine Burst. They also have access to a decent chunk of armor penetration through Acid Blade and a solid amount of Internal damage through "poisons" (though the damn names are all acid, whatever). The opening salvo of an Operative which takes your HP down to 30% is entirely composed of attacks which bypass Shields and Defense.
Mercenaries have the same attacks which can be defended against as we do plus Sweeping Blasters. While they are tearing your armor to shreds with Tracer Missile you are incapable of avoiding it through Defense.
Assassins have approximately 5 which these stats are capable of defending you against (the Saber attacks). Everything else not only bypasses Defense and Shields, but Armor as well.
Sorcerers, of course, will entirely bypass your defenses (unless they're stupid enough to swing at you). However I did find that their damage might be Energy and not Elemental as I'd expected. This may mean that your Armor is actually useful to them after all, and I can test this if there is a demand for it.
The classes we mitigate the most against are Warriors (both Marauders and Juggernauts) and Snipers. All three classes have several tools which bypass Defense and Shields, but for the most part their core abilities are affected by our defensive stats.
So, what does this all mean?
It means that, in PvP, Shields and Defense are nowhere near as effective as we've been led to believe. This further diminishes the general concept of PvP tanking and is a heavy blow to the Shieldtech tree especially.
This espescially diminishes the value of Shieldtech beyond Jet Charge if you do still wish to invest in that tree. It also confirms why myself and others haven't felt any more squishy after switching to a hybrid ST build.
PvP tanking still plays a role, however stacking actual tanking gear is not advisable. The main function of PvP tanking can be entirely achieved with Ion Cylinder and Guard. The most significant returns on PvP mitigation is from the Cylinder itself and the Expertise stat. Everything else you are getting an extremely poor return on.
This may have repercussions in PvE as well. We are currently theorized as the most mitigating tank (not by a large margin), however depending on what you can and cannot shield against in encounters we may actually be quite a bit behind the power curve (although our threat generation is still pretty slick). I have no means of really testing this at the moment, really no one does until we get some actual parsers or combat logs.
Unrelated to us: Warrior classes and Snipers are facing a pretty hefty handicap in PvP at the moment as they are the only ones significantly affected by mitigation/avoidance statistics.