Time and the number of tasks we're going to try to squeeze in, since we want 4.6 to come out relatively soon.
I'm not saying it shouldn't be on the table, I'm just thinking out loud that when the table's loaded with things we want to do and we get into triage mode tossing out things that can wait until after 4.6, doing an admin portal-based control panel is likely to get tossed onto the table labelled "the version after 4.6".
Maybe not, though. Clearly improving the admin UI ranks pretty high on everyone's list, including mine. If we decide to essentially rewrite all or most of it then I'd vote for writing portlets and a control panal portal page. If we just add some more complete links to the admin overiew pages and work on improving the worst pages then I'd probably suggest holding off on the portal approach.
