Since writing a blog post is much like writing something that would be normally done in a word processing progam like MS Word, I think that the composition interface should have more writing tools. A good spelling/grammar checker, a thesaurus, and the ability to compose within your blog's template (as in editing in a live preview) would all be valuable assets to the program. Text and paragraph control would be a nice feature to add in - suppose I want to left-justify a paragraph? Something like a word-processor formatting toolbar would go a long way to being able to write without having to worry about how the layout will look when it's finished. I don't need to see markup tags when I'm writing my post - how about just showing the final result?
So how about it? Should I look for it in the next release? :)