1) your blog will have to use their URL structure, /post/:id/:slug . But who is really bugged by that? Seems a sane structure to me.
2) fact. If disqus comments are a showstopper for you, you've got to find a different service.
3) fact, but I see that as a positive, not a negative. Why in the world do you want your themes running native code?
4) Tumblr has static pages with arbitrary URLs. Here's a screenshot of the interface: http://img.skitch.com/20100507-edeg31uwc25t9hkumww2j9hk4b.jp...
Tumblr has a great bookmarklet, a good iphone app, a good community, reasonable uptime, and a good admin and customization interface. I don't recommend their group blogs, though I have several, but I highly recommend their service for one person.
Good to know about the pages.
One other issue is file uploads. Say I want to host my CV at domain.com/CV.pdf. I don't think I could do that with Tumblr. Is that too much to ask?