I've outlined what I have created here: http://idea.kickofflabs.com/
If you are interested please sign up there or drop me an email at damaswill@gmail.com. It would be great to hear from you.
In the middle, people post comments on others' blogs or on meta sites such as HN.
I can't be sure, but from what you've said so far, I'm not seeing much unaddressed pain in the space.
As other said, there is a big expression space in the web, with room and options for everyone. But the fact is that, as you, many don't like the idea of starting a blog, or just want to be part of something bigger than what they are able to build. Anyways, figuring out exactly where in the map you will fit is very important.
Others forgot to mention old fashioned forums, that also allow people to post thing, open threads, etc. I imagine your product as something in between a forum + posterous easiness + blog network. What could help it a lot is thinking in a specific subject or niche, because that will make the sum of all posts something interesting to dive in (as HN is for some subjects and not others).
And of course, as michaelpinto said: go build a prototype as soon as you can.
Thank you though for your thoughts - it really helps.
It is a nifty idea to just be able to toss down some random idea on to a blog with no other pressure. With tumblr there is the pressure of having to blog regularly, queue up posts to publish every hour and keep to a theme etc.