I know the "upvote" and "mark as answer" should be used for this porpouse but, if someone answer my question and I like it, should/can I say thanks?
To quote the help page on the comment privilege:
When shouldn't I comment?
Comments are not recommended for any of the following:
- Compliments which do not add new information ("+1, great answer!"); instead, up-vote it and pay it forward;
That being said, if someone goes out of there way to make a really great answer and you say thanks, you won't get in trouble for it but don't be surprised if it gets deleted after awhile because it doesn't meet the guidelines for a comment.