Well...I guess my concern is the same for everyone else...performance!! On the performance end of things, well....I guess we won’t really know till we start coding it and testing it.
One thing I think we need to keep in mind is that the layout for the manager should really be open ended...meaning that the author of a template can not only change the look and feel but also the functionality of how it works.
For instance, if we parse the main menu items as list tags then a MTE author could create their own script and coding to create the functionality for that menu. I personally like the idea of a tabbed layout but not everyone would like this. Leaving out any javascript or such for the menu and instead placing it in the template code is much better and allows template authors more flexibility.
When it comes to placeholders, I think they should be customizable as well in the manager. The default placeholders (the main menu and such) obviously shouldn’t be removed and certain default items might be candidates for the ability to disable them should the user not wish for them to be used in any template. From here, a user can always create additional items via the use of snippets and whatnot...which is, of course, what we’re already talking about.

At least this way, templates for the manager can be zipped and shared by others on the main site without fuss about whether certain placeholders should be used in a template. If a placeholder is disabled, then the parser simply won’t display it or the div container for it. Simple!