Seems you're well versed on what the better sites offer; what more can you add to your own site to become the killer site within your niche?
Do you have any reference resources within your site? For example, an industry specific dictionary/glossary? Multi-lingual?
Many translators, interpreters and technical writers offer terminology resources and will link freely to quality content.
Some government agencies may list similar resources, as will some educational institutions and associations. All you might need to do is let them know that the resource exists.
How about product history or trivia? Product maintenance suggestions? Recall notices - including manufacturers' products offered by your competitors?