There are several challenges that a new entrant can face in the work place. But I will like to mention the top two.
1. Adopting the work culture: Work place is so different from school, you will need to unlearn, re-learn and learn the procedures, policies and process assets required to function effectively in your specific organisation.
2. Going the extra mile: In the workplace, there is actually nothing like, "my best is enough". You will need to always continuously improve in order to stay relevant and stay winning.