Mostly the hardware specs are better for the MMS at a lower price when compared to a similarly equipped non-server version. ($999 vs. $1049). Also the server version has dual hard drives rather than a single hard drive and it doesn't appear that you can upgrade the non-server version to dual hard drives (at least not from the Apple Store).
OS X server does three things:
1. Provides a GUI interface for controlling services.
2. Lets you upgrade the various services with Software Update rather than having to keep track of each service's upgrade path.
3. Provides compatible service configurations so you don't have to get various services to work together.
If you don't run one or more of the services that OS X Server provides, then OS X Server doesn't add any value.