They offered many features that wordpress.com copied (staging sites with one-button cloning; easy backup and restores; automated updates of php and wp code; tweaks to prevent security issues; automatic cdn) and others like serious engineers answering support tickets to help you sort out whatever fragile, insecure wordpress crap your marketing team, or their wordpress contractors, installed on your site.

We shouldn't overrate a lot of those features, because I think they were pretty obvious things to want. But WPEngine was, afaik, the first to market with all of the above in a pretty-cheap and seamless package.