The goal here is to avoid having packages end up in the situation
Magit is in now.
If ELPA inclusion is as cumbersome as Clement describes it, adding elisp *user* notifications will only upset users and will change little else.
The first thing to do seems to be seriously refine the technical process to have packages included in ELPA. Then we can improve the paper signing system, and then we can embark on a vast campagne to inform MELPA contributors that they can move to ELPA with little extra overhead.
If ELPA is concerned with free software, MELPA seems more inclined to think in terms of "open source software". We can't have people move to free software if oss proves to be less of a hassle (the minority of people who can easily be convinced that free software is better are probably already on ELPA).