If you look at screenshots online, you'll notice there isn't any UI/health-bar/etc. . Instead the game provides you with visual cues as to how your health is doing performance wise; such as your armor disappearing (think Ghosts N Goblins with more layers) or the top of the screen turning red.
There's also some "boss fights" where whether you win or lose, the story progresses the same. The only benefit of winning is the boss has less armor when you face it's final form. From the reviews I've read, this seemed to off-put people.
One big complaint I've read is that the combat is shallow, in the sense that once you learn all the initial attacks, there isn't any further development in technique. The game is like a juggling act between the 3 weapons, where you can only hold one at a time, and have to steal from an enemy to switch. I actually really like it. Mastering each style of combat keeps gameplay from getting repetitive for me.
Platforming can be aggravating, especially in trippier environments but I've certainly dealt with worse.
The 2d side-scrolling stuff handles really nice, and is a good change of pace in-between the 3d levels.
