Are you sure you're playing the class correctly?!?!
I've been level 50 on my sage now for a few weeks (rolled it as a new toon because I got bored of sentinel) and I initially thought I was going to be a healer but for some reason, DPS just seems more fun to play.
I'm rocking a custom 2/21/18 tree, have approx 1700 Willpower and Endurance (unbuffed), I'm wearing full NON-BiS WH with half Fortitude Augments and half Resolve Augments, I've got around 29.3% Crit Chance and a 79% crit multiplier (unbuffed) and I've managed to get my Power stat to a reasonable 890.
On the server I play on there are quite a few awesome marauders, juggs etc and I can manage to survive and burn them down with no trouble at all, even if I'm being piled by quite a few of them.
I average 300k+ dps per WZ (which is alright considering I don't have a pocket guard/healer) and can still manage to play WZ's objectively and earn a fair amount of medals.
This was a few days ago in my full WH: http://imgur.com/P5hm0
This was when I had full BM: http://imgur.com/Ibbx0
All I say is, you just have to manage your rotation well and know how to play your class during different situations. I'm not saying that I really know how to play this class 100% but hey, I'm getting a fair idea of what to do.
The DPS and survivability on a sage is awesome if you kinda know what you're doing.