There are two styles of pant fronts; those with pleats and those without. There is no hard and fast rule when it comes to pleats vs flat front pants. The pleat is certainly more traditional and will always be so, while the flat front is more stylish.
Pleats came about in the late 1800s and arose more out of practical considerations than anything else. Gentlemen used to wear their pants closer to the natural waist than sitting on the hips. This made mobility a bit more difficult with trousers particularly when hunting or doing other activities that we are all now accustomed to doing in our dress pants. As a result of this, pleats were added to provide more room inside of the pant while maintaining the waist size. Pleats were and continue to be useful for men who are of larger girth.
During the time of the war, much like cuffs, pleats were seen as an excessive waste of fabric at a time when everyone was rationing everything. As a result of this they became less and less popular. The flat front pant became prominent in the 50s and 60s had has solidified itself in modern day fashion.
If you are a thinner gentlemen looking for pleats, try to stick to no more than two pleats as the addition of any more pleats will not compliment your structure. To this day, there is still some formal attire that is designed with pleats in mind. A classic example being a traditional morning suit that should have one pleat. The flat front pants are more of a continental European evolution whereas the pant cuff is seen to be more Anglo America. As a result of this those who really know their fashion know that it is against the rules to wear a flat front pant with cuffs. That said, some rules are made to be broken. We'll save that for another day.