The success of the Roman army was based around the facts that it had excellent organization, great leadership, good training, good equipment, and that is was only consisted of professionals.
The Roman army had only professional soldiers in it. Not everyone was able to become soldiers, as certain physical criteria had to be fulfilled. The ones that became soldiers had training everyday. Tactical training, one to one combat, formation combat etc.
That led to the Roman army to be very successful in its military campaigns, especially because very often it was coming upon poorly organized armies, where most of the soldiers were either part-time soldiers, or just ordinary villagers.
