The front door will make a lasting impression on a potential buyer because it is one of the first things that they will notice. It is like a smile on the face of the house, and it should be welcoming. If it needs to be painted, use a modern color that matches the color palette of the neighborhood. If it is stained but weatherworn, it should be re-stained. As with the entire house, doors should be clean and in good shape. Look at the other houses in the neighborhood to determine which type of door you should have. If most of the neighborhood has original wood doors, stick with that. If most of them have painted white doors, paint the front door to match them. Make sure that you take a look at the wooden door frames around your exterior doors. They are notorious for absorbing moisture and then rotting. If you find that you have frame problems, repair or replace them now because a home inspector will definitely discover the problem.
New doorknobs can update the look of a door inexpensively, as well as fresh paint and kick plates. Make sure that all of the locks lock easily and that the hardware is in good condition. Tighten locks and hardware if loose. If you find that some of the doors in the house seem to be difficult to close, it might be because the hinges need to be tightened. Clean all of the glass on the doors whether it be stained glass, leaded glass, large sliding panels or small window panes... they should sparkle. Here's another place where a Mr. Clean Magic Eraser will come in handy... use them around all of the doorknobs in the house. You will be amazed at how clean your doors can look. You might even be able to save yourself a coat of paint or two.
Finally, if the insulation around your doors is cracked and peeling, just replace it. It's a cheap fix and one that will definitely be noticed by buyers looking at homes during very warm or very cold months.