I'm working on a jQuery-based one right now (with a matching Drupal module, which would make it light-weight, as Drupal bundles jQuery). Once you understand the nuances of designMode/contentEditable, things get easier, but I know what you mean about how much you need to consider. It might feel like wasted time when CKEditor 3 (smart name change, in my opinion), which looks like the real deal, is released on December 1st.