I'm not sure I understood everything you said.
How do you determine a user is an idiot or not when good ideas look like bad ideas initially? Experts can have blind spots.
The option for reputationless discussion should always remain open in my opinion.
> how high is the cost really?
The cost can be proportional to the proof of work.
Everything has a cost. How about determining the cost and providing means to pay for it? Paying works for many things in the world. It works for ads, for example.
Why must the cost be annoying a human by requiring reputation instead of a monetary cost? How is annoying a human a better solution than letting people pay? Do you really think pissing people off will stop them from expressing what they want? It might be more likely that people will express what they want, nobody will hear them, and they'll leave, taking good ideas with them.
Just as there is a cost to moderation, there is a cost to losing good ideas. How about letting a free market decide?