Advice on Giving Feedback?

Hello Everypony!

As a new member to the community, what kind of feedback do authors tend to look for, or appreciate here?

In terms of background, I give feedback on a regular basis on DeviantArt artwork,
so have a fair idea of things like: don't troll, spam, or generally write unhelpful comments.

However, I am interested in what other people consider good feedback!
I really am interested in getting feedback on my feedback XD.

For example, is it appropriate to give spelling and grammar corrections in the comments section?
For stories that receive a lot of comments, I feel like these would clog up the space.

Any other general advice for a newcomer also appreciated!
(I have read the FAQ, just in case someone suggests that ^^)