As I've played a Sentinel upto rank 44, I can honestly say we're not underpowered. We are, aside from Scoundrels, the best in a 1v1 situation in Watchman/Annihilation. I can regularly take 2v1 and win, 3v1 and occasionally win, 4v1 and kill 2 and 5v1 I can kill a couple and survive long enough for support to arrive.
I do however, feel we are the most gear dependent class in the game. We don't have a prolonged stealth, we don't have a knockback, we don't have a castable stun. Outside of our CD's, without gear, we're easy kills. Once you have gear, you do become something that needs to die quick.
Also, people don't quite realise that we're a support class, not the main damage class. We bring Zen(Watchman healing), Transcendence (Speed boost and 10% Melee and Ranged defence) and Inspiration (15% Damage/healing Increase, stackable) to make us invaluable in team fights.