I first began critiquing stories when I joined an online young writer’s critique group at the age of twelve (i.e. nine years ago). I’ve been critiquing ever since, and I enjoy it so much that I’m actually hoping/planning to intern with a publisher or literary agency next summer. 🙂 If I can. *crosses fingers*
As for my style, I am a ridiculously thorough and detailed critiquer. Not even kidding. If you ask me to critique a piece of fiction, I will give you reams of feedback; I’ve been known to write a 2,500-word critique for a 4,000-word chapter, or 19 single-spaced pages of crit for a 50,000-word novel (though these are extreme examples). I think this is because I:
a) try to explain my thoughts as thoroughly as possible in order to make sure that what I’m saying makes sense, and
b) like to offer plenty of examples and suggestions on how to improve things.
Thus, I don’t generally mark things in the manuscript itself, preferring instead to type up my thoughts in a separate document. I also do my best to be kindly and constructive in the comments I offer, and I always try to include some compliments mixed in with the critique. 🙂 I like to think I give the sort of critique I’d like to receive, but as I’ve yet to meet anyone who crits the way I do, that’s just a guess. :-p
As for the type of critique, I’m pretty flexible depending on what sort of feedback the writer is looking for, whether it’s about plot, character, style, consistency, tone, rhythm, logic, or all of the above. However, I admit to being a bit of a Grammar Nazi, so left to my own devices, I do tend to give notes on more nitpicky stuff unless I’m specifically instructed to do otherwise (also, if there are too many grammar errors, I may get overwhelmed and stop marking them).
I also second what a couple of other people have said: I think it’s important to do your best to help the writer write the story s/he wants to write, not the one you think it ought to be.