Creating a full blown 3D game requires a lot more math for physics, but what if I just want to mix 3D models in a 2D game, such as in the New Super Mario Brothers? Would I be able to get the looks of 3D models with the ease of making a 2D game?
Given that most "2D" games are still in 3D engines, just using camera-aligned quads and an orthographic camera, not hard at all.
Simply replace your standard quad with animated materials with an actual model that animates and keep your logic the same.
Here's a tool that does just that - takes 3d models and turns them into sprite sheets, including rendering all the sprites for the animations:
reasonably priced too!
(caveat: I haven't personally used this tool, just read the review at: http://www.torquepowered.com/community/blog/view/16207/2 )
It's not uncommon to have tools that will take your 3D models and render them out for your game. Many studios and indie devs write up their own tools to do it and makes generating the graphics super easy. It's also nice to be able to go in make a change to your model and then just hit a button in your tool to quickly re-render all the raster graphics.