As I understand it, with Tumblr you can't customize your URL structure, you need to use external commenting systems, and your themes can't run non-js code. Also, many people use Wordpress as a hybrid CMS/blog; the ability to have pages (again with a defined url structure) is important.
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.
llimllib, thanks for the reply. I can think of a number of reasons that you might want to have dynamic elements in your theme. I understand the concern over themes running code, but if you want things like breadcrumbs and your CMS doesn't come with them as a native function then you need to be able to run native code. Ideally, the CMS would include a wide and extensible array of native functions that themes could take advantage of, but tumblr certainly isn't that CMS.
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?
