Things to keep in mind about the end product when designing:

• Small features will get blobby and not dimensionally accurate
• Corners are rounded because of the circular nozzle:

• Surfaces are generally rough in one direction and smooth in another because of layer lines
• Overhangs will not print cleanly since they are unsupported by a layer beneath (support can somewhat mitigate this)

Feature Rules

Tolerancing

Check out 3D Printing Tolerancing for more information.