One thing I've found extremely difficult to handle is the Zookeeper cluster model of containers. Where when a thing dies, a thing has to come back and be able to referred to as "zookeeper-1" forever. The way to do this currently is to use a service in front of a replication controller with one pod. This feels wrong all over. Supposedly they have a thing called Pet Sets [1] coming to solve this, but it's been in the works for an eternity. Also we've started to outgrow the load balancing simplicity that the k8 load balancer gives you, and I have not seen a nice migration path to something like HAProxy in Kubernetes. All that said, we like kubernetes a lot.

[1] To distinguish from cattle. If you have a red blue and green goldfish, and your red goldfish dies, you can replace with another red fish and not really notice, but if it's purple, the others won't play with it.