One major difference (like already mentioned) is that I wanted to use Dockerfiles rather than Procfiles. I wanted to have full control over how it built and ran - and I didn't want to go through slugbuilder / buildstep at the time.
It may actually be easier on end users to use Heroku buildpacks and abstract some of the magic - but for our uses Dockerfiles were the best fit.
Now, the Tutum team came out with something that even simplifies it even more - and it might be worth looking at:
http://blog.tutum.co/2014/04/10/creating-a-docker-image-from...
I have had that on my backlog for several months and just haven't had time.