Scripler seeks to simplest user experience possible. According to project lead David Birk, it's designed so that anyone with rudimentary knowledge of word processing can make a fully functional and good-looking ebook.
A user can either import a word file or build an ebook from scratch, section by section and chapter by chapter. They can set templates, add metadata, insert the cover, and more. Many of the standard components of an ebook, including the title page, copyright page, and TOC can be automatically generated from the metadata and text of a book, saving authors the hassle of crafting one manually.
And when the ebook is finished, a user can output the ebook in Epub3 or publish it online.
Scripler is still in the alpha stage of development. It is currently free to use, but the developers plan to offer both a free tier and a paid premium tier.
It looks like it will be a useful tool one day, but it's still missing a few things. I think the different menus could be better labeled. I'd also appreciate a list of keyboard shortcuts; as anyone who uses a word processor a lot can tell you, having to use the mouse slows me down.
But that's just me. Have you tried Scripler yet?