I feel like that kinda goes against the spirit of it. Posting about be permissionless but not costless.

faradayfedora

You'd never see new users stuff. New users would never get off the ground.

A blacklist however, everytime you see a "botpost" blacklist em. Should help weed out the crap.

Public_N_M_E

This could be mitigated with a trust / whitelist type system.    If I meet you at a conference or just generally trust your posts are not bot/AI content then I can whitelist you.    Make it a setting to toggle on/off the whitelist such that once you have enough whitelistees you can keep it on and enjoy the smaller-scale discussions with people you trust. 

drdoom

Link spam

SN's biggest risk is downward spiral of post quality and bottification.  Whenever money is involved it attracts the greedy and unscrupulous