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.
Check it out: http://ec2-50-16-106-77.compute-1.amazonaws.com/ - (it's still in its infancy)
You can link the same post as a reply to multiple items. This allows for complete flexibility. Posts that are relevant to more than one section can live in each of those places
I think that this was one of the most important strategic decisions in reddit's history, and that they got it right.
I'm not saying tags can never work, just that any proposed tags system needs to supplement, not destroy, the siloing of subreddit communities. And be simple to use, even for the 99% of redditors who never even vote or subscribe to anything.
I'll happily admit that tagging could not have grown Reddit to anywhere near its current size without the site collapsing on itself. The different feel to each community (compare F7U12 are AskScience) is much more appealing than a single homogeneous group. However, I think that it was the first step towards breaking the promise to create an personalized news aggregator. I for one was disappointed when the recommended posts feature was dropped.
The initial missteps with whitelabel sites like the Wired-branded reddit and lipstick.com are amusing in hindsight. I'm not sure how reddit with a pink background with Courier as the primary font was supposed to attract a female audience.
For example, if a post could be tagged "startups" and was posted to r/business, when I tried to find other subreddits besides r/startups about startups I could search "subreddits with x or more stories tagged "startups"" and I'd be presented with r/business.
They wouldn't exist as a replacement for subreddits, they'd exist along side and serve as a way to connect subreddits by topic. Subreddits currently exist as their own entities with no crossover which doesn't work well for expanding a users subscriptions to other subreddits relevant to their interest.
That said don't listen to me. I have quit using reddit, except for /r/gonewild.