Then you will gather those skills based on what's interesting to you, not based on what's marketable to a startup.
I have never gone out of my way to learn a computing skill specifically for a job (or even for a "dream"). All was always out of personal interest. By that method, I have accumulated a skill set (including Ruby/Rails programming) that has allowed me to start my startups when the opportunity was there.
You should be wary of even being a programmer if your primary driver to learn new things is what might be useful to your career. All good programmers I know and know of would be learning and applying their programming skills whether or not there was money involved.
I agree with you, but I don't think it invalidates the OP's question. There are dozens of topics in computing I could learn right now that I would be interested in, and I think pretty much every HN reader could say the same. If some of those would help their career and others wouldn't, I don't think it would be fair to blame them for learning the one that would help their career.
I'm sure if Paul Graham had asked an equivalent of this community what he should learn to increase his market value, "Lisp" wouldn't have been the answer. At the time, probably C++ or Perl. Yet he used his knowledge of Lisp to gain a competitive advantage for his company, and it ended up making him rich. C++ and Perl may have helped him too, but that's actually less likely, since they wouldn't have given his company its competitive advantage of nimbleness and rapid changes.
You can't know in advance what will be worth learning. Except, you can: if you want to learn it, if you have a desire to understand how it works, and if that desire is storng enough to spur you to learning it, it's worth learning.
I'd add an extra indicator actually: if everyone tells you it's worth learning, chances are it's not. If everyone tells you it's a waste of time, chances are it's worth learning. Why? Because people in aggregate will point you towards popular options (duh) - and those will probably give you less of an edge. This makes the OP's question even less valid.
I have never gone out of my way to learn a computing skill specifically for a job (or even for a "dream"). All was always out of personal interest. By that method, I have accumulated a skill set (including Ruby/Rails programming) that has allowed me to start my startups when the opportunity was there.
You should be wary of even being a programmer if your primary driver to learn new things is what might be useful to your career. All good programmers I know and know of would be learning and applying their programming skills whether or not there was money involved.