What makes a SaaS product stand out?

I guess being original, having the wow effect, or at least giving a new approach to the product, by targeting a new market, audience, or profile such as marketers, developers, founders, makers, etc. or by focusing it in another way, to agility, profitability, efficiency, etc. Or by offering features that were missing in the existing ones. For example, we at RealMail focus email validation to SaaS, because they have their own peculiarities, which others overlook or do not understand.