It's a walled garden in the sense that you can't just grab any old software from any old source and install it, and that Apple can pull an app if they feel the need. The wall is pretty low but it's there.
Is this a good thing? That's the real argument. For the vast majority of users the answer would have to be yes because they're downloading software from a trusted source. It doesn't eliminate the risk of malware entirely but is shed load better than the roulette of downloading from whatever random site you find with Google as for Windows.