currently it's not very intuitive that the open files are are under the window menu... i like the paint.net ways of editing multiple files: previews of pictures and a drop-down to show all open pictures.

I do like the paint thumbnails too for files overview.

This may be already implemented in master branch according to wishlist bug report: https://bugs.launchpad.net/pinta/+bug/671449

Pinta has long been able to open multiple files, but it has been hidden away in the Window menu. In Pinta 1.1 there will be a panel (like layers etc.) that you can have open to show all open images. Not quite as fancy as Paint.Net, but very much useable.