I feel it's the speed at which you can move your person on the screen. If you were playing on the 360 or in the Arcade the movement would be at a set speed but in this it's as fast as you can drag your finger. Still it's amazing to have this game in the US for only 5 bucks.
im no good at shooters (still love them and buy them though) and even i find this a little easy. i got ot level 5, normal difficulty on my first go which for me and my level of skill is unheard of.
but the sheer quality and number of explosions going on all the time keeps me coming back for more. i actually like it and play it more because its easier, i think i would give up in frustration if it were harder.
graphically its amazing. wonder what it looks like on the 360. bet it looks amazing on that thing.
definitely will get any more cave shooters that get released.