MMC is just the console, you have to tell it what snap-ins you actually want to load.
If you do a search in the windows\system32 directory for files with the .msc extension, those are all the basic snap-ins included with windows. compmgmt.msc is the Computer Management, services.msc for the Services management screen, etc.
The .msc extension is registered to MMC, so you can just make a shortcut to a particular msc file and it will open automatically.
And of course you can write your own console snap-ins.