In a comment on my last post, Matt asked several exceedingly thought-provoking questions.  I was going to respond there, but I had too many thoughts for a single comment.  I had to take off my hat because I was getting too warm while thinking about this…
  Ah, but what to pick, Mr. Panozzo?
The road less taken?
The thing that you love?
The thing that really could work?
The thing you can make a living at?
The thing you're rather disinclined toward but you're good at and know will have the greatest impact?
Knowing you can pick a couple, sure, but you yourself admitted to 500 possibilities. Maybe one of each, or something that answers more than one question?
What'd you pick?
These deeply resonated with me, as I have thought along similar lines.  As an example, in October in some personal notes I wrote the question "What is the highest good man can accomplish?"  It seems like some of the questions were along these lines, asking what is the best or highest thing that one can aim for.  Can you make the best impact doing something that you are especially predisposed to do, or something that you really just want to do?
As far as "what'd you pick", I'm not sure if this should expand to "what would you pick" or "what did you pick."  Either way, haven't quite gotten there yet.  :)  I'm still exploring these questions myself.  With that said, I don't have answers, but here are some thoughts.
Viewing the questions and possible answers through the lens of existentialism applies almost perfectly here.  There is the knowledge that you have limited time and that ideas cannot be implemented without using some of it.  There is uncertainty in the value of a decision.  What will it will be worth to you, and what will it be worth to other people?  What are your weights for these values?  There is angst because you realize that you are absolutely free to choose.