1. The situation would be the same as it is with Debian and Ubuntu at the moment: both use DEB but the packages aren't compatible. Only difference is that they would be even more incompatible between various major distributios.
2. New users would still have to download packages for their own distribution
3. Propietary software vendors would still have to provide different binaries for different distributions (in most cases at least).
4. The packaking would still be done by distributions so there would be absolutely no difference from developers point of view.
Your second point has nothing to do with the RPM/DPM debate. The horrible user experience has nothing to do with this either. The package management is already abstracted using PackageKit and therefor it doesn't matter what package manager backend you'r using and the interface can remain the same. The AppStream project tries to solve it but again it doesn't have anything to do with this.