Thank you everyone for the responses. Rather than quote and respond to each post individually, I figured it'd be simpler to put in one post since many of you identified the same issues.
I agree that the drums need some work, the kick in particular. I'm thinking I screwed up the position of the kick drum mic when I was recording as I just couldn't get the "click" sound of the beater that I wanted. I tried fixing it with some eq, but I think the only real solution would be to record it a second time.
I was torn between adding more guitar parts and keeping it simple. I really do not know much about mixing (or recording, or songwriting...) at this point, and so I'm attracted to keeping things simple. Less voices clashing, less worry about trying to make sure that everything can be heard. However, there's no arguing that simplicity leads to less variation, which in turn makes it less interesting. I hope that as I continue this I'll naturally grow in more complicated arrangements.
Again, thank you to everyone for donating your time and thoughts.