The point is that both are platforms. Application developers shouldn't really need to care about the internals of a PaaS, for the same reason that I don't really care about the internals of the Linux kernel.
I've been using Deis (http://deis.io/) which is built on top of Docker and inspired by Heroku. I think it's a bit more "lightweight" than Cloud Foundry but really pleased so far. I had a few issues but the Deis team was always quick to help and fix bugs.
If you want a lightweight Cloud Foundry, try Lattice[0]. It's specifically intended to allow developers to experiment with the core Cloud Foundry components (routing, scheduling/placement and log draining) with very low overhead.
