The other thing is his only complaint (slowness) isn't really a problem. If you have a critical hot spot in your server code, extract it, implement it in C / C++, and use ctypes to call it.
* Experience with Python, and general experience in duck typed environments.
* Discipline. Not just your own, but anyone else working on your project.
There are certain mistakes that will make your life miserable down the road. These could be bad organizational conventions, inconsistent exception raising/handling policies, lacking a strategy for documentation, etc. If you make mistakes like this, you probably won't get bitten by it until later.
Writing good Python can be harder than writing good <X Language with more rigidity>. With experience and time, this gets a lot easier.
I appreciate all the niceties that dynamic-typing language like Ruby provides. But I am not sure the trade-offs are enough (all you already described).
C has become the lingua franca of computing. You can call everything (except Go) from C and you can call C from everything. So you want to actually reuse standard libraries, you will want to know C. And if you're a good C hacker you'll know C++. I'm not saying you'll necessarily use it, but you'll know it.
The vast majority of python programmers do not know how to call C/C++ code and are therefore limited to pure python libraries exclusively. They usually use writing to files and os.system() as an alternative. Needless to say, the performance of this approach can be questioned.