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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?
Mint 17.1 Cinnamon HP Pavilion 17 Notebook, 8GB DDR3, 2.5GHZ A10-APU, Radeon HD 8650G
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.