Something for those of us for whom the job is just a way to pay for what we actually want to do.

Oh, I was supposed to put down my job? :( Oops.

No worries, it's not clear at all what you're supposed to put down. Personal focus, professional focus, aspirational focus, academic focus, general interests, hobbies...? I'm not sure. The list provided isn't really a good fit for any of those things.

I thought whatever comes to mind first and I think that suffices to get an overall picture. I wanted to keep it as simple as possible. Maybe I should have added a short explainatory text at the beginning? How would you exactly improve the design of the questionnaire?

The weakness of the poll is in part a reflection of the weakness of your question. What is a focus? Why is it a main focus? What is the question you're really trying to answer?

Explanatory text is good when it wouldn't bias the responses. Explicitly allowing multiple selects is good to be stated up front (see ColinWright's explanation elsewhere). Using an "Other" response is relevant when you don't intend to change the possible responses halfway through (and changing them effectively invalidates your results up until that point).

HN is the only place I've seen get regularly irritated by poorly-written polls; the Internet is filled with really bad ones, because survey design isn't taught unless your major actually needs them (i.e., user experience, social sciences, marketing and sales).

I appreciate your clear and upfront response. I shouldn’t have submitted this poll without giving substantial thought to the question and options first. I only wanted to get a very rough picture of how the main interests/hobbies/professions are distributed on HN, so I didn’t care much about rigor; but that probably negatively affects pollee morale. I’ll keep that in mind for the next time.

Physical sciences? (instead of physics)

Done.

Programming Languages?

Is it too broad for a subcategory of theoretical computer science?

I'd personally say it fits in theoretical compsci.

Operations Research

Do you think it justifies for a separate category? Wikipedia says "It is often considered to be a sub-field of mathematics.", but I know that Wikipedia is not always correct and that I have some other small categories already at choice.

It is a bizarre conglomeration between CS, Mathematics, and Industrial Engineering. It is distinct from math in much the same way that CS is distinct from math.