Experienced in most sorts of writing, whether academic, journalistic or creative. Though I'm not experienced as a betareader, I would probably behave much like your average editor: grammar, spelling, storyline suggestions and corrections for any character/plot issues or factual errors. I'll be harsh if need be, but I can also make good suggestions to go with complaints. My basic-level Japanese, Spanish and French experiences can come in handy, though I definitely can't beta fics written entirely in anything other than English. (Just if you have little quotes in those languages... or something... I can check those. XD)
If you want my help, I have a few basic criteria.
1. Follow basic grammar/spelling/syntax rules so that I can at least understand what you're trying to write.
2. Don't be a drama llama. (In other words, if it turns out you can't take my critique, don't turn around and flame me on your Myspace.)
3. Don't blindly take/copy & paste my advice/corrections. Yes, I generally know what I'm talking about, but if you have a question or concern regarding something I say, do ask. I won't mind explaining a grammar or overall story concern. :)

Grammar and spelling. I'm no PhD, but I've developed a reputation as a grammar-Nazi-in-training. I'm also good at spotting Mary-Sues and cliches, but can recognize if and when they are used in fresh ways or in ways that are truly beneficial to the story.
Vocabulary issues are typically easy for me to spot as well, especially with insanely similar words (like "effect" and "affect.")

I'm rarely good at spotting OOC-ness, and this sometimes occurs as a weakness in my own writing. I can also be a little harsh, and will likely show some bias against fanfics from Twilight or Maximum Ride, unfortunately. You have been warned.
As for grammatical weaknesses, I'm poor at spotting misplaced modifiers, especially in my own writing. I have other writing weaknesses, but I can usually spot those just fine in other writers' works. Heh. :)
I will also likely be somewhat slow during certain parts of the year (mid-December, end of May, other exam times) as I am still a high school student with a heavy courseload and an anime blog to maintain. I'll try to give you adequate warning or go on hiatus/vacation mode if a particular time is a bad time to submit anything to me. Regardless of schedule insanity, though, I will always try to get a story back to you within 3 days.

Preferred genres include romance and drama, though I'm cool with comedy and other stuff too. Preferred fandoms are basically any non-shoujo anime or manga series (with some exceptions) or NCIS.

No Twilight, Maximum Ride or those little boarding-school-drama-llama-cliquey-rich-whore stories unless it's something you truly think I can handle better than any other betareader you know (which is not likely.) I doubt I'll be great at action-loaded stories, since I can't write those very well myself. Too much angst is probably a no-go for me, though I don't mind it in moderation.

All Anime/Manga All Books

All Games All Misc

All Movies All Plays/Musicals

All TV Shows

General Romance Humor Drama Adventure

Mystery Horror Parody Angst Supernatural

Suspense Sci-Fi Fantasy Spiritual Tragedy

Western Crime Family Hurt/Comfort Friendship