It now makes sense why they stepped away from the Job DSL Jenkins plugin that they helped create a while back (though there's still support for Jenkins steps in there which'll be a great help with porting existing pipelines).
This feels like a Jenkins replacement rather than a Jenkins plugin replacement or at least something that sits atop Jenkins in driving your build and deployment activities (it's got support to call and watch Jenkins jobs so it's sensible to assume they use both).
I'm interested to hear that JobDSL is painful for Jenkins masters do you know what the pain points are?
In a multi-tenant environment that we run using Enterprise Jenkins from CloudBees, the default option for JobDSL is to create jobs at the root level which is problematic if you don't allow teams at that level. We've also had issues with jobs just getting "stuck" which even after a restart of the master doesn't clear it out. Plus the fact that it encourages bad behavior among teams by allowing them to create thousands of jobs that never get cleaned up is just another reason we've banned it from other installations we've setup. Jenkins itself is pretty terrible and even the proposed 2.0 changes don't appear to do anything to change things for the better. If Spinnaker can replace Jenkins, more power to them. But I don't think that's the initial aim.
Regarding your other comment about "something that sits atop Jenkins in driving your build and deployment activities" that's just about exactly what the Workflow plugin ecosystem does. However you're still stuck with Jenkins and its baggage.
Jenkins is also a first class stage type - it is the way we run the build and test side of our deployments
You can basically do all of this in Jenkins today, but you don't have the fancy GUI, that's my impression so far.
http://spinnaker.io/online_docs/overview/overview.html