Posterous is a great platform for publishing (and autoposting). The main inconvenient is that it's not free software and there is no way to host it yourself (though you can use a custom domain). So the goal here is to see how Wordpress could be tweaked to work as (much as) Posterous.
A short list of Posteorus features that I find useful and would need in Posterpress.
So Wordpress support tags already and has a bookmarklet for a quick posting from any webpage: http://codex.wordpress.org/Press_This
So for the other features, here's a list of wordpress plugins that might help or do the job.
And once you've configured it all, all you need to do is import what you had in your posterous and save it in your wordpress.
Just some stuff not directly related to a posterous conversion.