There is no discernible through line connecting all of December's Games with Gold. It's a scattershot of old and new, all of it from completely different genres. Puzzle, platformer, fantasy RPG, and over-the-top shooter. I guess that means there's a good chance most people will find something that suits their tastes.
Microsoft has told us which titles make up December's Games with Gold stocking stuffers. On the Xbox One front, explorative puzzler Q.U.B.E. 2 is free for the whole month. On December 16, Never Alone gets the nod. You might remember Never Alone as the beautiful little platformer about a girl and her fox that tells a piece of Iñupiat folklore.
However, it's on the backward compatibility side that people might find the most value. From December 1 through 15, BioWare's Dragon Age II is free. From December 16 through the end of the month (and year!), Mercenaries: Playground of Destruction takes over. Mercenaries: Playground of Destruction is a well-regarded gem from the days of the original Xbox. We'll see if it still holds up.