Thanks for the suggestions!
Art isn't my strong suit, so I just made enough to make the game feel a bit more polished. Visual variety is definitely something I'm planning on adding at some point, but I wanted to get a good grasp on how the gameplay (and maybe story) would work before I did too much on the art.
Since I mostly focused on solid gameplay mechanics, I'm very glad to hear that you don't have any complaints about those.
On the topic of speed, I don't think that it is necessary for the game to be fast-paced. As I work on the art, I hope to develop an atmosphere, so that there would be something to look at and take in as you think.