Major differences I can see (OP feel free to correct if I'm wrong):

Link.fish

* doesn't provide a web crawler

* relies heavily on microdata, schema.org, RDFa, etc

* relies on manual parsers for sites that don't have microdata embedded

* doesn't full-render pages by default (Diffbot renders every page, so it can use computer vision to automatically extract the data)

* doesn't support proxies

* doesn't support entity tagging

Probably plenty more, but that's what jumps out to me at first blush.

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Since I see other people have mentioned price as a concern, we're always willing to help out bootstrapped startups. Just shoot me an email: dru@diffbot.com