Having a large experience with Debian package building it's refreshing to try something else. Last week I have learned the basics of RPM package building.
Here are the differences I have found so far and my opinions about them.
The RPM .spec file package contains both package metadata (description, dependencies, etc) and compile rules while on DEB you have the data split into different files. I remember that on the beginning it was hard to understand the purpose of all those debian/* files, I have found .spec files easier to understand.
The RPM .specs allows conditional building, during build the build target release can be used to dynamically adjust build flags, dependencies etc. While you can achieve this on Debian using some auto generation mechanism (debian/control.in), it is not naturally integrated in the building system, debian/* contains metadata and rules for a specific target system.
Not so important but a nice feature it's the support for translated description/summary on RPM packages.