The door itself should be solid with a solid wood or steel core. A lightweight door that has a wooden hollow-core construction would be a lot easier to breakthrough. Its also important to make sure the door and surrounding framework are in good condition and not suffering from deterioration in the form of rotting, cracking, or warping.
Naturally, the doors lock plays a crucial role in how secure it is. It should be fitted with a good quality deadbolt lock. You might also want to think about a bump-resistant lock as bump keys are commonly used by thieves nowadays. A door chain can be used as an extra security measure but never as a substitute for the deadbolt lock. A peephole is another good option as it lets you see who is at the door before opening it.
If the front door has glass paneling in it make sure the lock is as far away from the glass as possible so that if the glass were smashed, it would be harder to reach.
Occasionally a door will have a mail slot in it. In this case, there should always be some type of cage or box surrounding the slot on the inside of the door. If not, it may be easier for someone to access the lock from the inside through the mail slot.
These are just a few ideas to show the type of characteristics a secure front door should have.