Well, the overworld seems pretty much 3D, considering how you go up hills, and your cannonballs react to gravity on a 3-dimensional plane.
However, when controlling link, you interact on a 2D plane. Even when climbing up ladders or vines and stuff, it's still 2D, like in aLttP. The difference between this 2D and the 3D in OoT or WW is that you actually interact with the Z-axis, like TBR said, whereas this game uses a forced camera angle most of the time.
So, I wouldn't say that it is really "2.5D", but in fact switches between 3D and 2D. And the 2D that the game uses simply uses perspective to make it seem as if it is 3 dimensional, even though it is actually just 2D.