Can any anonymous visitor Up-vote or Down-vote Q/As?
No, anonymous user cannot up-vote or down-vote at all. To be honest besides adding question and answer he cannot do much.
In the best case scenario everybody should register and be active part of growing community. It is just better for a q&a owner, but we all know, that the world does not work like that.
First of all anonymous user is just wrong name, it has a negative ring to it and nothing to do with anonymity. It is more like Guest posting, without all that register crap. That way You can add one question (if You want to add just one), wait for answer and forget about it afterwords.
Now, as a Guest You don’t even receive mail about new answer to Your question, You cannot comment, up-vote or follow, to know if there is some activity etc.
There are many people who do not want to register and will give up from any contribution if they have no other way to participate.
What is considered as “do much” ? Asking and Answering is allowed (optionally) and the only thing is comment missing.
This anon thingy is ubsurd imho. Why somebody ask something anonymoysly, without reg i mean.