The most difficult (and most necessary) part of completing TBS is learning how to handle different personalities. By 'handle' I am not simply implying how to be nice when communicating a plan of action and intent to your peers. That is easy. The difficult part is developing the ability to reach out to a peer who has clearly 'checked-out' and surgically navigating around his sensitivities (e.g., alluding to his lack of mental toughness) to motivate him to accomplish a mission-critical task for the unit.
Correcting peers is hard. At TBS, you are forced to correct your peers systematically (rankings, peer evals, etc) to the point of negatively impacting the unit's cohesiveness. However, the evaluation process drives the retaliatory tendencies, pissing-contest-type out so that by the end of TBS, students have an excellent ability to evaluate peers/systems/ideas objectively without letting personal/comfortable feelings interfere.
Another extremely difficult part of TBS is trying to break students out of their fear of acting without explicit guidelines. OCS shatters self-confidence and individualism. Thus, after OCS, the new officer naturally gravitates towards the group-mentality because he fears correction as a result of doing something wrong. However, our doctrine is built on implicit communication and decentralized execution. TBS is designed (albeit, not perfectly) to bring the self-confidence back that OCS destroyed without the individualism that hinders the team-player mindset required for the job.
That is why the Marine is f'ing better than everyone.
The best part about TBS? Hmm.. "we're not sure, so just standby for now."