I've been leveling since level 10 as Darkness and I loved it. This being the first class that I've leveled as a tank and actually enjoyed the tanking. Right now my current spec is something I fished and pulled together.
Having a shorter CD on Force Speed & Force Shroud and a longer duration on Force Shroud means a lot more mobility (important to a tank) and survivability aswell.
There's no "should be" in the world of ratings. If you choose tank gear, then you'll need to take what you're given, although once you start getting a lot of gear, you have the options of playing around with mods n so on.
Basically, you should be taking all kinds of willpower based tank gear, starting with having a shield generator instead of a force focus.
Ideally, you should have this distro : Defense = Absorbtion > Shield > Willpower > Power > Crit > Surge > Accuracy > Alacrity.
Getting the survivor setbonus from tionese/columi/rakata/campaign gear is also important, 5% more shield chance is a lot.. and the 4set bonus is awesome aswell.
There's no real soft caps or "get X to value Y, then stop" for tank stats - more is better. You'll find that it's very easy to accumulate shield rating and while this isn't bad, choices with more defense or absorption are better. However always choose the highest armor rating gear (or armor mod!) because the rating of the gear determines armor, which in turn means more damage mitigation.
Accuracy should be avoided if possible, since it only really affects our white melee attacks = Saber Slash/Trash/Assasinate. Shock/Wither/Discharge and the rest are Force attacks = 0% miss chance. So when you start getting your rakata gear with enhancements like end/acc/def or end/def/abs, replace them with columi end/shld/def or end/shld/abs enhancements.
I feel that Assasin tanks are some of the strongest tanks in game if you play to their strengths.. 'Sin tanks have a little less overall raw mitigation, but thanks to the fact that we need 0 Accuracy, we can gear for more tank stats. We also have the highest hp of equally geared tanks when compared to juggs/PT's.
Also our "snap" aggro generation (solo and aoe) is really high and the rotation isn't the difficult.
Combined with Force Speed, Force Pull and our 2 major survival CD's we're in a pretty sweet spot.
Also, just for fun, it's possible to solo all the lvl 50 daily missions as an assasin tank + healer comp with a little bit of gear, even the 4m hc ones.. and yes, also the Corellia one since you stealth/sap/force speed through all of the packs.
This is the main reason why I love playing a 'Sin tank : The stealthy solo play and the kickass hard/nightmare mode tanking pro'ness
Just to show what's possible these are the stats from my 'Sin tank (Full Rakata with some Black Hole/Campaign pieces) - fully stimmed and buffed
Health : 25171
Armor : 7003
Defense : 345 rating = 26.39% in total
Shield : 513 rating = 63.84% in total (including 20% from temporary buff)
Absorb : 310 rating = 49.07% in total
0 accuracy, crit or alacrity.
That means my total stat budget is 1168 rating pts - ideally that would leave me with 467 defense and absorb and 233 shield rating.. but because of the shield rating heavy gear, this is what it's like. At the moment, the only real valid course of action for me would be to go down to Columi enhancements or something which would be quite a downgrade if I wanted to drop shield and gain def/abs.
Defense/Absorb - secondary option with these is to get augmentable gear and put the defense/absorb augments in them. Its typically 10-13 points per augment, so 3 or 4 augment pieces, and you'd be in good shape.
The problem is that you can only have 1 orange main piece if you wanna keep the 4set bonus which I personally think is too good to not keep up.
Of course, if you have campaign tier set tokens, you can take out the armor mod and put that in an orange and keep the setbonus, but as this was meant for help to a newcomer, I didn't think it was too relevant.
But yes, the End/def/abs enhancements are by far superior, but there's not that many of them around because of their highly sought-after nature.