There is a huge difference between simple and lacking in features, yours falls into the latter category. Something like Tumblr (although full of features) is "simple" because at its core is the ability to do everything you need for a blog to do in a few clicks, but if you need to do more complicated things you can, that makes tumblr simple.
Yours is literally 2 features, creation of blogs and making posts. I don't see where the market for your platform is, if I want this why don't I just use HTML files on shared hosting? Same features + a custom domain, lower cost.
Tumblr also shares Brytter's feature of providing a consistent experience for readers. While you can visit an individual blog which will have it's own theme and unique elements, most Tumblr users are reading in the dashboard, do designing your theme is not a huge deal.
Tumblr and Posterous are so good, not to mention Wordpress — I really wouldn't want to get into the blogging market at the moment.
Yours is literally 2 features, creation of blogs and making posts. I don't see where the market for your platform is, if I want this why don't I just use HTML files on shared hosting? Same features + a custom domain, lower cost.