It has integrated CI whereas GitHub requires using a third-party CI system, it's had Reaction Emoji (called Award Emoji for GitLab) for a while now, it has "Merge when build succeeds", CE is entirely FOSS, etc.
GitLab and Atlassian have a pricing model for displacing existing systems and encouraging adoption, but I can't see GitHub following suit. GitHub looks to like they want to be the Apple in the Apple vs Android. Android has the largest user base but due to Apple's premium pricing, they have the largest profit.
Its quite puzzling why gitlab went that route, they are basically discouraging ci systems from integrating with them. I think this is would turn out to be a bad decision for gitlab.
As a third party service provider you could still host and/or modify those CI runners for specific tasks like node, go, etc.
And to give some additional background to dominotw's question. We didn't like the options for CI on-premises. And by integrating it directly into GitLab it is very easy set up a new project, encouraging people to use CI on more projects.
Having people use GitLab CI will mean GitLab is a less attractive platform for other CI products. We do have a good commit status API since GitLab 8.1 and there is a great Jenkins plugin that supports that http://doc.gitlab.com/ee/integration/jenkins.html