Management/orchestration. It's really convenient to be able to deploy things in different docker containers, and isolation, and etc., but the actual orchestration seems to be unnecessarily painful. In no particular order:

* k8s is apparently ridiculously hard to set up if you want to use something that isn't AWS / GCE / GAE / non-"big-cloud"-provider-here.

* Many of these have questionable (IMO) default authentication choices. I can understand if it's running on an internal network or something, but if you can't set up an internal / private network to hide the management interface (ex Marathon, k8s dashboard, ...) inside, you're SOL it seems. Ex. Openshift Origin having a login UI, but the default allowing logging in with basically anything you want.

* Rancher seems nice, but between the internal DNS service randomly failing all DNS queries until a full restart and rancher-server's internal MySQL server writing to the disk at 500Mbps, it's... iffy. At least it's a nice UI/UX otherwise and supports lots of authentication options out of the box.

* Straight-up not working. For example, I tried running Openshift Origin on stock Debian and Ubuntu installations, and it couldn't even finish starting up before it crashed and burned. I did file an issue against openshift/origin about this, but so far it's been unresolved. Another example with Openshift, after figuring out how to work around it failing to even start up, it can't even create pods/containers. This is apparently a known bug that's been around for a while (~4 months) with seemingly no resolution.

* Weird docker version requirements. I can get that things like k8s would want to pin to LTS versions of docker. That's fine. But if I can't even import the signing key to install the LTS docker version because it doesn't even exist in the keyservers anymore, that sounds like an issue to me.

Maybe this would be different if I was using one of the Big Cloud (TM) providers. Who knows. But with my budget of "poor college student," OSS offerings are all I can do when my entire budget is consumed by server bills - and I can't cheap out here; a few TB of bandwidth and maxing up to 16 CPU cores isn't cheap :( - and it just seems like the "state of the art" is pretty terrible, at least from the UX perspective. I spend enough time staring at text and delving into the CLI and whatnot when I'm writing software. Why is it seemingly so hard to get even a simple UI to cover the basic functionality - create a master node, add slave nodes, deploy containers, and scale them up? Rancher seems to cover this use-case the best, but I've run into enough issues with it that I'm starting to seriously consider figuring out how to write my own orchestrator. I'd rather not, since it'd be a lot of work, but setting up these "standard" tools is a ridiculous Herculean task if you don't have a massive budget, it seems.

Ninja-edit: For a bit of perspective, I'm talking about a spare-time project that ended up gaining far more users than I ever expected. I'm the only person working on it, so trying to figure out all the development, ops, etc. on my own has been a huge struggle since so many of these tools just have a god-awful UX.