It struck me that in my case, part of the reason I don't blog much is that I'm so often on a mobile device and not a computer. If it was easier to blog from my phone, I just might.
This was part of what I liked about Posterous and Posthaven, both of which specialized in posting via email. We all have email on our phones, so it removed one of the biggest barriers.
I'm not sure what the ideal solution is here, but blogging from mobile is something that all blogging platforms need to address (hint, hint).