I've just gotta reply to this... I have a 50 sentinal, and a 35 guardian... and I've never had any of the issues mentioned in the OP, I found that with my Sent, I can pretty much run anything over far quicker than they can bring me down...
Golds of same level, I will typically finish with 60% - 70% health, with my Guardian a little less only because I'm not killing them quite as quick, although I'm sure that will be changing as he gets higher levels as well...
But, as another poster mentioned, I took my Sent into a bunch of Heroic 2+'s and solo'd them... now if BW want's to make the Knights more powerful, that's fine with me, just saying that I've never felt underpowered, or had any trouble completing any PVE of the same level, and usually 2 or 3 levels above...
Oh and BTW, Sent was combat spec from lev 10, and Guradian is Tank spec