I personally think it would be a really good idea to allow voting on instant answers without being signed in. I think the site would get more input if this was possible. Several times when I've visited the site I was going to vote for an idea, and realizing I was not signed in lazily abandoned the idea... I know it's lazy, but I'm sure I'm not the only one.
Also people who have just visited the site for the first time are not likely to create an account unless they have a more significant contribution to make.