Let me explain for those who don't "get the AT" why it works. One more time...
Versatility - you can choose ranged or melee in a combat situation, dare I say it? Situationally. If someone else is overwhelmed, I can hop in and save their bacon. If no one else is around me, I can dive head first into a mob, and be whittling down the boss as it's standing there amazed it's mob is suddenly ashes. I can swap team roles in a heartbeat, and do exactly what the team needs from me at that moment. Be that damage dealer, soft controls, or alpha strike absorber. And then do something else in the next moment.
Aggro management - if I'm not attacking it, it's looking elsewhere, period. Sentinels peel off aggro like a Stalker going into hide. When solo on a Sentinel, I can easily manage exactly how much aggro I'm taking on at that moment.
Mitigation - oh, is that a problem for other ATs? I never noticed any mitigation problems on my Sent.
Unique tweaks - all the primaries and secondaries have really interesting little tweaked powers in them that other ATs don't get. Fire/ has complete lack of movement suppression, /SR gets master brawler, DP/ gets additional range with cryo ammo, etc. etc. Read up on these, find an exploit that sounds profitable, and go to town. For example: My fire/bio is indeed a more steady damage dealer than a fire/anything, and never gets mezzed. I haven't even finished the build on it yet and it feels pretty boss because of the movement non suppression, the uptime of the nuke, and the repel baked into the snipe.