The discussion on this ticket has actually touched on two subjects – an error in the DB upgrade (i.e. missing civicrm_mailing_ab) and the integration with Drupal WYSIWYG. Reactions on both:
1. I've taken the latest code and compared (a) the schema for a new install with (b) the schema for an upgraded site (4.5.0 => 4.6 circa revision cb113651a5) and confirmed that they have all the same tables, columns, and constraints (using SHOW CREATE TABLE on all CiviMail-related tables).
2. As a bit of background: One of the major goals of the CiviMail UI rewrite is to perform a trial-run at replacing some old, EOL parts in the system (HTML_QuickForm, Smarty v2) with more modern, widely-used parts (AngularJS). Unfortunately, the configurable WYSIWYG stuff is deeply tied into HTML_QuickForm and would need to be rewritten/reconceived to work with Angular – and this hasn't been done yet. Consequently, CiviMail in v4.6 is hardcoded to use ckeditor.
Using Drupal's WYSIWYG is a neat feature (e.g. when uploading files through the WYSIWYG's buttons, the CMS can provide its own file-manager), but it's also fairly complicated and makes it harder to add other features (such as the mail-merge token UI), and it's actually one example of a more general problem (ie revising widgets in Civi based on the CMS). We could try to rush out a work-around for v4.6, but I suspect we'd do a bad job on the v4.6 timeline. Instead, the hope is to keep this simple for v4.6 but – in the next version (4.7/5.0) provide an API which could make this customizable.
The CiviMail interface has changed a lot in 4.6 - might want to check and see if it's still a problem in that version (check the online sandbox).