If I want to make a non-4-sided PCB (talking about the limitations), then how can I use the polygon tool to create for instance a hexagonal PCB?
Step 1 - check with your PCB fab house and see if they support non-rectangular boards.
Step 2 - draw your outline (0 width line) on layer "20 Dimension".
I've used radiused corners on almost every board I've sent to OSH Park. It's a nice touch, makes it look like you know what you're doing, even when you don't. 
-j