I'm not 100% sure what I like best about sentinels but I recently started a Beam Rifle/Super Reflexes one and it's some of the most fun I've had in City of Heroes. Not my first 50, but this character quickly became the closest thing to a main I've had. I have played a Water/Bio sentinel previously but that vharacter never really took off.
First I like Beam Rifle, but I like BR on my BR/Temp blaster as well, and that character is fun but not this fun. The survivability of sentinel /SR is a big part of it. Toggle status protection and a clicky absorb is nice, even though I am that guy that likes clicky status protection. I really like that absorb heal combined with defensive opportunity. Opportunity might not work great with some secondaries but an extra 40-ish hp/sec plus endurance boost on a /SR toon is nothing to sneeze at. It's nice to hit a big target with it but just having it active is pretty huge.
Round out with the psi melee attack and the psi PBAoE and I'm happy, and I even took those for concept reasons with no idea if they were good.
I guess it boils down to being hardy enough not to have to set things up or anything. Nuke is up every 22 seconds or so and that's usually enough to mop up if I'm not on +4. It's not quite the melee experience, but it's close and I get to shoot people with an FGMP-15 and mind bullets.