Radians are the hidden core behind trigonometry and can be a little tricky to understand at first. But once you get the hang of it, you will never want to go back to measuring angles in degrees.
Trigonometry is one of those things that makes people run for the hills, but once you understand it, you won't know how you ever lived without it.
The dot product is one of those elusive things that you aren't taught soon enough (In my opinion). If you want to do any lighting calculations yourself, then the dot product will be used extensively for diffuse calculations.
Vectors are the back bone of 3D graphics, without them, the world of computer graphics would be a very messy place. And knowing the length of them often comes in handy, especially when looking at intersections and collisions.
Lines, who doesn't love lines. They start at one point and end at another. In fact, lines are essential. What makes up a triangle? 3 lines. How do we draw a circle? A lot of lines. Knowing about them would be a good idea :)
Pythagoras theorem is essential if you are doing anything that involves triangles. One of the building blocks of mathematics.
The distance formula is one of the first things you learn once you get into maths. It is fundamental, especially when it comes to collision in computer games.