So what's my bear of the day? Slopes. Going up and down uneven ground can be a surprising challenge to implement.
Unity uses an implementation of Box2D, so everything is floating point numbers and vector math. For any artists out there, it's an awful lot like switching from Photoshop to Illustrator. Your art sensibilities carry over, but you have to tie your brain in knots to get the tools to behave how you expect them to.