How come I lose reputation points for downvoting?
Is reputation sort of like a "currency" and that it "costs" me reputation points to downvote an answer?
I currently upvoted back that answer, since I don't have that many points to squander :(
Here's the FAQ on how reputation works. The one-point cost is simply to discourage massive downvote campaigns. Honestly, you should ignore it... it's more worthwhile to both yourself and the community if you upvote good stuff and downvote bad stuff. If you're concerned about reputation, give good answers to questions over time. If you're not concerned about reputation, then who really cares anyways? :)
Do note that voting on Meta works differently.
Yes, reputation can be interpreted as a sort of currency: you earn some by asking and answering with quality. You can award bits of your reputation for bounties, or to draw more attention to a specific question. Since knowledge and science are cumulative, I understand why positive actions result in a reputation gain (or none), to encourage quality. However, downvoting should be taken with care, and I believe the point loss is quite symbolic. Alternatively, you can edit the post which, if accepted, may provide you with a little reputation gain.
And the first downvote gives you a
critic badge. If a question or answer poses serious problems, you also have the possibility to flag it. Useful flags also earn a badge.