I do agree with phwd's viewpoint that one does not need to post +1 comments to express their opinion. It remains that it is a common practice on the sites, and I do believe there exists an answer to your inquiry. But I think a 'good +1 comment' is not so much an entity of its own as it is just the opposite of a 'bad +1 comment'.
A 'bad +1 comment' does nothing. All it does is establish "I upvoted you", which is clearly done by the actual upvote. The bare points of "I agree" or "I think this is useful" are also expressed by the upvote itself. All of that information adds nothing to the post, and it's just taking up space. This is roughly equivalent to "me too" comments. You can accept on your own questions, and you can vote on other people's posts before you can comment on them. So use that privilege if all you wish to do is express gratitude or an effectively voiceless opinion on the usefulness of the post.
The opposite of a 'bad +1 comment', then, does something and adds to the post. It'll explain why the post deserved an upvote, or provide supporting evidence that proves the veracity of the post, or bring up a related bit of information that people would find helpful, or expand on the principles that the post provides. It doesn't matter how substantial it is, just that you're saying something more than just "I upvoted this". It can be as simple as "+1, very comprehensive answer".
I feel that '+1' is not so much a class of comments on its own, it's just a small thing you can tack onto any comment that you post. If it is your style to put these on your comments, then simply check whether your comment has any business existing without the +1. If it doesn't, then it's probably not a good or valid +1 comment.