Interactive or not
As Michael Jenning's comment "Philosophy, not just design" points out, the difference between RPM and DEB is not in the source code to build the package manager.
RPM is more suitable for the newcomers and busy sysadmins, while Deb is more suitable for enthuasist who enjoys tweaking around. I've used both types of package manager, and personally I prefer Deb.
The interactive option may be a bit annoying at times, such as when I installed it at 4am and went to sleep, I found it was asking me to configure when I woke up, but it serves as ways to configure things that would set up the system once it finished the installing the packages. For example, when one installs MagicFilter it would scan for a valid /etc/printcap and see if it can get things working, or else it would ask you for configuration. This may be bad news for the sysadmin who's trying to install 50 machines, but if the sysadmin have the time to sit down and finish such questions, the system would be running when the configuration is over.
RPM has a funny dependency engine that when I used to use SuSE, it shows up some funny problems. Such as when I was trying to install a Gnome specific program, it ask me to install "gnome-libs", but I was already using gnome...... How could I run gnome without the libs? Isn't that inviting segfaults? I've no such problem with apt-get to install, in fact I don't need so much time to go to sites like here to hunt down rpms.
Deb is not perfect either. Since RPM dominates the user base (hey, there's way more people using rpms than debs), maintaining packages is harder as there's not as much hands to put things together. I found myself getting licq-0.76 while the latest is 0.85. Also, the interactiveness of deb doesn't always happen as there was only a handful of package asked me to configure at install and none ask me to configure once I had a running system.
One feature (If there's one, tell me please!) that I would like to see in rpm and deb is that the admin could tell rpm and deb that a tarball is installed; therefore maintaining the dependencies properly. Currently, licq is not registered in the deb database as I installed the latest tarball.
Merging rpm and deb is a great idea, because I believe that rpm users wanted apt-get while some deb users may want the silent, non-interactive rpm.