I have noticed jumping from Halo: Reach's forge environment into Halo 4's takes a little bit to get use to. I love the new magnet system. Its awesome... Well kinda awesome. I think that instead of magnetic points on each object, each object should have magnetic rails. I find myself having to add piece on top of another piece just to get the magnets to line up. I think have the magnetic "points on the corners only, and have the entire side(s) of the piece magnetic.
Trust me, it will help out a lot!
also.. IDK if it is just my forge menu (before you enter forge world) or if others are having the same problem, but there is two scroll bars for the menu. one is where it should be, the other is off to the right about 2 inches in the middle of the screen. Problem still there after xbox restart.