What about making your idea conditional to incorporate points made by ThatFanficGuy: only posts with 'full hubwheel' and responses to them can be read by anyone, other will need you to register. I think that it would both allow lurkers to see quality topics while still making most spam to seem like a waste of time that it is.
Perhaps add a bit of logic that only users who got their first full wheel and are able to add community tags can 'bump' the thread to visibility. That way spammers making a dozen accounts to self-promote would not be able to game the system.
But it's a good idea to try for a while regardless.