We're planning on trying to grow a more mature community than the ones you see on Tumblr with a focus on actual content and the authors rather on the users.
Tumblr has a plenty mature community — and one focused on very serious issues (Tumblr helped surface the Trayvon Martin story, for example). I know college professors who post there regularly. There are artists who do amazing things there (see: http://annstreetstudio.com/). They would probably benefit from a customizable approach as well.
I think this is a mistake and you may find it hard to find users willing to submit content because you're not giving them much encouragement to post.
EDIT: Upon reflection, this statement was a bit harsh and I tempered it a bit, but I do hope you consider ways to focus on the user as much as the creator.
I apologize if my comment came off as brash. I've had limited experience with Tumblr but my bias is the result of me taking a small subsection of it and extrapolating the (not great) experience with it to the whole of Tumblr.
To address your second point, we're now thinking up ways of how to engage users more into posting content on our site.
How can you build what is essentially a subset of tumblr and then say you have little experience with tumblr? This would strike me as not being very serious about your show hn. At least look at the competition thoroughly so you can find an angle that you can differentiate your product by. The lack of mobile support is insane, and the lack of customization would to me make this an under thoughtout idea.
