Honestly, beyond being stalked by patrolling enemies, the inability to fight back, and having a finite refuelled light source, the similarities are extremely few.
I really can't stress just how fluidly Outlast plays... I was gobsmacked from the first few moments. The feeling of
being the character is so well conveyed, and the interaction with NPCs is something that was glaringly absent in Amnesia (and still appears to be in A Machine For Pigs), but Outlast nails it.
Best horror game I've played in a looooong time (and I'm a horror journo -- believe me, I play them all). Also got A Machine for Pigs pre-ordered, so I've got Outlast to scare me senseless this week, then Amnesia for next. Be still my beating heart!