First I realized not that much. SEO is about evolving anyway so anything that changes you CAN adapt to . But anyway saying I feared nothing is no fun so here are a few that I probably wouldn't normally say I feared but would cause me some concerns
1) Well obviously for me if Private networks no longer worked. I can't see how that would happen but that would be concerning
2) If Google decided to expand the ads at the top of the organic column pushing down organic results two or three spots. Yeah that would lower conversions,
3) If negative SEO REALLY could bring down any site.
Thing is........ so much of what SEOs feared has already happened in the last two years that theres not that much left. Keyword data being shut off, PR not updating, all kinds of links not working etc etc.
But whats your SEO "fear"? and do you think its about to come true?