If a story had tags and there was a system where the frequency of tags appearing in a subreddit mattered it would allow me to look at r/startups and then find the other subreddits relevant to my interests.
reddit made the mistake of treating every subreddit as its own individual isolated community without considering crossovers in interests. If tagging existed then this would not have been a problem. Today 6 years on it's still impossible to find good subreddits relevant to specific interests, tags would have been one of the solutions for that.