That's a hard one....but I'd say the very rich but flexible features set and the awesome technical support
Nothing so far. We are in the process of setting up or site and testing, So far, nothing has been an issue and where we needed support, it was easy to schedule and spot on.
Nothing I can think of
We needed a replacement for Crowdbase which is turning off their platform at the end of the year. After a through search for a replacement, Knowledge Owl came out as the clear winner on every one of our requirements. First and foremost, we needed the ability to mimick the look (to some degree) and interaction behavior that our users had become used to in Crowdbase. KO provided this. The KO feature set is actually overkill for our needs, but very solid and not so granular that a none coder can't get things done without support. However, when you need support, KO REALLY shines. They are a relatively small organization, but growing, so they can really provide personal service. If KO can find that magical balance of growth (which will come just based on how great a platform it is), and continue the quality of service, they will own the knowledge base space.