Most people don't really like an extremely nice beta-reader that doesn't give any useful feedback. I think that I could give you constructive criticism that might improve your story, and I'll do my best to word it nicely. :) (Because I'm also aware that people don't like very mean beta-readers as well)

What you need to expect from me is that it might take a while for me to understand your writing style. I'm not a very narrow-minded person and I'm aware that we each have different styles of writing. I might like some and I might not like some. If I'm only so-so with how you write, I promise to tell you that you should probably get a different beta-reader who will give better criticism since I'm sure we all wouldn't want to read a story with a writing style we don't appreciate. (Though with a book series that shall remain unnamed, I didn't like the writing style but I was entranced by the lovely plot and action scenes, another device you could use to grab my attention.)

From what I have observed with myself, when I judge a story, I first pick out the grammatical errors and then move onto the context after I've done that. There may be some lines I'm iffy with even if they seem correct and I'll point those out to you, too.

Since I'm a reader like everyone else, you have to keep me interested as I read. If I find that it's dragging, then I'll tell you to speed the pace of the story up a little.

With whatever I edit, I put general comments at the bottom of the thing that basically summarizes whatever I observed about your writing style and things you might need to improve on.

Grammar is basically my forte and I've just recently started becoming better at picking out characterization issues. I'd say that I'm pretty good at putting in the right amount of description, too.

Action is definitely something I'm not good at, I sometimes have a hard time expressing a complex plot

Comedy (I don't appreciate crack, though), Romance & BL, LIGHT drama and angst...

About the length, I appreciate long stories, but only ones that are long because they need to be. I don't like extremely long stories that don't have any substance (For example: a story that's freakishly long just because of the descriptions of the landscape and the characters). At the same time, I hate reading stories that cannot occupy more than one line per paragraph (In short, I don't like short paragraphs).

Regarding fandoms, PM me and we'll talk. :D

Please do not give me completely mindless stories, complete with a very unstable plot and a Mary Sue/Gary Stu that everything revolves around because I think every fandom has seen enough of those. This is typically what fanfiction is defined (and is completely completely wrong, may I add) as and I don't wish to cause you any grief by accidentally saying my negative feelings.

Also, I'm not into gore, but if it's really nicely-done gore, I'm all up for it. :) Just a heads-up for you guys.

All Anime/Manga

Romance

Humor

Drama