If it's my story, I want that error gone, regardless of the type of error it is. I want to know about it and I want to fix it. Whether I'm notified publicly or privately, I don't care =P Criticism to me is a good thing -- it means I made somebody care enough to comment, and that means the world to me.
When I read what others have written, however, I'm different. If it's a characterization issue, and I just can't get around saying, "OMG Clark would *never* do that," or, "That is just *not* Lois," then I will not read it. Plausibility is an absolute must for me. If the writer can't make me believe something then I simply won't waste my time reading it. If someone can create a completely off-the-wall premise and make me buy it that means more to me than an unbelievable story with no typos and correct commas at every turn.
Grammatical stuff I am a lot more lenient with. As long as I don't have to waste a lot of energy trying to figure out what is being said then I'm fine with it. This is fanfiction, and a lot of folks tend to write as they speak, not as grammar would call for -- and that's fine with me.
There are some general basics that drive me bananas when people can't do them correctly -- like using they're, their, and there appropriately, through, throw, and threw, too, to, and two, knight, night... things like that... "A lot" is two words. It's is not possessive. Etc. Those are the only things that really bother me, although it takes a lot of occurrences before I will stop reading.
I try not to give corrections, however, unless I know the person writing well enough to know how they take criticsm, or when it's specifically solicited. People who write to this extent are usually writing for self-enjoyment in my experience, and if criticism will ruin that, far be it from me to contribute.
