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I'm guessing that the December LMDE release pertains only to those either wanting a 64-bit version or someone just starting with LMDE who doesn't want to install 350 updates required by the September release. Everyone who has maintained their LMDE installation since September will already have the December release, right?
If you've been doing regular upgrades, then the new version will be just more upgrades. You'll see the changes, such as they are, when you restart your computer. About all I've noticed is a new splash screen.