I couldn't chose just one, so I'm promoting the concept of ranked choice voting (if not the first then the second, etc):

1) decentralizing with go1dfish

2) crossposting to multisubs and/or applying metatags
- allow modifications after posting
- allow your friends to tag your stuff

3) site-wide notifications
- in a little section above or below the submit buttons
- curated by admins
- maybe with a dedicated sub to discuss suggestions

4) flag/mark and/or read/unread
- to help better sort out where you've been and where you want to come back to
- could be similar to metatags

5) improved search and/or improved navigation
- more options than going back through history of pages one page at a time looking for something months ago is impossible

See also: /s/IdeasForSaidIt/wiki/feature_ideas