> Maybe the answer is that they need a invite of some sort to surpass the "reputation threshold".

That's the only plausible idea to me too, but it doesn't solve someone hearing "nostr" on twitter and googling it and trying to get onboarded. 

 yep, it doesn't. Maybe, just maybe, that's a feature. U know, invite-only socials have a sense of exclusivity. 

 Unfortunately the applied solution will be "please type these letters from the image" unless we figure something out 

 yeah, but I am not that pessimistic.

It's an order of magnitude better to do chaptas ONLY ONCE and then rely on your acquired reputation, than solving them all the time 

 Captchas don't really solve anything either, they just make spam a bit more costly, but people will resort to them without anything better on the table.