I expect if the other threads on this subject are anything to go by is that all you will get from the numerous prima donnas on this site is a barrage of abuse.
So me start the ball rolling from a non-pilot's perspective; most common factors thus:
1. Runway blocked by departing/arriving aircraft in front
2. Cabin not secure for landing
3. Aircraft not in a position from where a safe landing can be completed (e.g. too high, fast or displaced by strong crosswinds)
4. Minimum visibility requirments not met at decision height (e.g. low vis approach where weather drops below minimums)
5. Some kind of equipment failure on final (e.g. landing gear not showing as down)