What pros or cons should a beginner be aware of when deciding between a 2d game engine (like Slick2D/Flixel/FlashPunk) and a 3d engine (like Unity) for 2d game development? I am just getting started in indie game development, though I have dabbled a bit with Game Maker, Flash, and XNA in the past. I've heard a lot of positive things about Unity, and its cross-platform nature makes it appealing, but as I understand, it's a 3d engine at its core. For a strictly 2d game, are there any compelling reasons to work with a 3d engine like Unity? Or would it just add unneeded complexity to my initial learning experience?