Congrats on taking on MarsEdit. I look forward to seeing what you're going to add and improve.
I'd love to see a way to add photos to a post from iPhoto or the filesystem.
This is the one thing that ecto does that works really well for me -- it uploads two versions, a thumbnail which is included in the post, and the full-size photo, which is linked to from the thumbnail in the post. Very simple and very handy.