I clicked on one of the ones which said "Time's up!" and it asked me for a password. So perhaps they are also password protected.

Like you would give the password to someone who you wanted to read it?

The most glaring flaw is that this service isn't necessarily going to exist in a month, year, decade or century.

All password protected drop has a little 'lock' icon on the top. Well, I'll try my best to keep the service running as long as possible. But I agree with you, it requires a lot of works to keep any services running in a long run.

It depends, there are 2 types of drop - public and private. Public drop will be available to everyone. Private drop is always password protected. 'Quick drop'(non logged in user) is consider as a private drop. You can only create a public drop once you created an account. :)