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Well one of the biggest differences is obviously privacy. A phone conversation has an assumption of privacy, and it is in fact illegal to listen in without permission. Therefore, anything said on a phone network is entirely attributed to the speakers and doesn't reflect back on the phone company.
Tumblogs on the other hand are public by default. In fact, I believe the only way to have a private one is if it's linked to a public one. Anything that's said on Tumblr can potentially have an affect on their brand. For example, many people dislike Reddit for not policing "their" content. I wonder if this policy extends to the private blogs?
I definitely agree that privacy is part of the equation. But (like you suggest) it must not be all of the equation, right? If Twitter started banning self-harm promoting tweets, I think there might be some criticism. Somewhere between Twitter and Tumblr lies the line for most, I think.
What is that other part? Size/format of content? I wonder, does Blogger have the same policy? What about Posterous?