Are you required to sell out domains under a gTLD? It'd be interesting to create some sort of must-be-a-sub-entity TLDs like ".bank" which requires it members to be banks (similar to .edu and .gov). But I can't imagine any entity but ICANN itself managing that in a 'fair' manor.
The registration limitations can be arbitrary. For example, I was excited to see that my city had signed up for a domain, but in the application they say the domain will be closed from public use and that they will delegate names only to official departments of the city.
But I guess if you are planning to make a commercial operation around a gTLD, its best to let anyone register.
It's like a job application filter. Start working for a .int company, only pay 10% income tax (but still get all benefits everyone else gets, sometimes more).
