The biggest mistakes made by beginners starting out in landscape photography, are in composition. In my opinion, it is one of the most important things to get right "in camera". Composition (aside from a little cropping) cannot be fixed in post processing. Nothing you do on the computer is going to change how you arranged the scene in the viewfinder.

Tweaks can be made, of course - e.g. cloning out intrusive branches or straightening the horizon but generally, composition must be right at the time of the shot.