[megameter]

The most important thing from both the production and audience experience standpoint is for the game to feel coherent: it does not leave open contradictions in its premises and execution. If you ignore this the scope of the project easily blows up ten or a hundredfold because the contradictions turn into an unsatisfying experience that is very difficult to solve without resorting to brute force effort.

If you have that, though, you have avoided the biggest source of failure.