Reddit has tried this approach and, IMO, it's failed. A new human user will spend actual time creating a thoughtful and helpful post, only to be greeted by "sorry, your post has been removed by automod because you don't meet criteria". They get disheartened and walk away forever. The spammers, on the other hand, know how the rules work and so will just build their bots to work around this (waiting 30days, farming kar…
this is the reason I never was keen on StackOverflow etc
tried posting there several times, many times actually - every time some annoying condition was not met
well screw you too then! walked away and never bothered to contribute again