ZF is from a totally different league than CakePHP or Wordpress. Yes, a bit ancient today but in many parts it's a solid, well-written, flexible and extensible code well-suited for more "enterprise" needs. Probably more "production-ready" than Symfony2 is at the moment. Plus: ZF2, which will require PHP5.3, is in the works so it should make the project more "relevant" again.
Didn't dwell too much into Lithium's code but saw they have some interesting ideas also based on 5.3 features, e.g. they use a sort of aspect-oriented programming based on closures and lambdas. Sounds cool, even if only for experimenting with different programming approaches.
