Well I think multiple items need to be done to sins and soon, first one possible the damage reduction shield sorcs have.
Two they are a cloth class and expected to partake in melee, so correction this should be first but they seriously need to evade/reflect a lot more like dare I say 98% of the other games assassins out there. A mega glass cannon atm later in the game, strongs and elites just tare you up if they lay a hit on you even running dark stance you wind up companion dependent in deception tree which is one of the most fun trees. Easiest fix would be a passive skill added in later levels granting evasion or some sort of damage reduction.
"Solo survivability passives are needed" in the dps non tank trees or in the commons for assassin not the 5stam bs given atm that's just a really a sad week way to go, this is because no matter how much stam you stack on if you get hit like a brick against a wet paper towel every time by adds or other players it means nothing and wasn't thought out.
Hardest fix would be to give assassins leather like a marauder gets leather instead of plate and adjust the armor mods according for the assassins.
More frontal burst possibly later in the trees possibly a talent tree add-on to put a point into allowing you to have maul 360deg of your target not just from behind. Key point being in pvp or pve not everything wants to allow you to sit behind it smashing away where a marauder/jugernaut/Sorc/agent etc can stand toe to toe and do continuous high dps no matter where they are standing on you, just feals like your hacking threw a tree limb with a butter knife atm trying to balance your stuns and cool downs in pvp and pve to reposition, less of an i sue with madness on that one but lets not end up like Wow forcing DKs to be unholy for best longevity because face it its what a assassin is atm but a hell of a lot squishier than any other class out there....
I know I have a high lvl sorc and it's like night and day compared to my 50 assassin I can solo silver/golds/and champs on a sorc given time and khem, where as solo a strong I can rock but end up low health on a sin. I have only rolled with the tank trees and Decep for the most part was not as much a fan of the Mad tree to much necro/lock melee wanna be there, less melee instant gratification, more of a annoyance for pvp because any class that can damage reduce or slight heal can brush them off, I roll threw them in tank or decep tree before they deal enough damage to take me down.