I've never beta'd before, but I'm a professional writing tutor IRL, so....yeah? The English language has been my sandbox for going on 20 years and Supernatural is my current fandom of choice, so here we are.
One note, before we get started: Okay, here's the thing – I'm not through with the series yet. So far I'm about five episodes into Season 6, but anything after that point is off limits for me. That still leaves five whole seasons, though (plus a good twenty years pre-series), in which I would be glad to lend my support to any writers who want it.
Genres you will beta:
Action/Humor/Drama/Fluff/Kidfic/Gen/Crossover (provided I'm familiar with the other fandom – for instance, I'd totally be up for Superwholock, if there's anyone out there interested in writing it). My favorite kinds of stories are either hurt/comfort or slice-of-life-type things. Wee!chesters are awesome.
Genres you won't beta: Slash/most Wincest/Het/Threesome/Mpreg/Deathfic/PWP/Smut/BDSM/AU/RPS/Genderswap
Pairings you will beta (within your specified genres): Dean/Lisa, Dean/Castiel, Dean/Jo, Dean/Anna, Castiel/Anna, Sam/Jess, John/Mary (this isn't an exhaustive list, just the ones I could think of off the top of my head). A Note on Wincest: I cannot and never will ship it, but if you've got a Sam/Dean story that fits with all my other parameters, go for it.
Pairings you won't beta (within your specified genres): Sam/Lucifer, Sam/Ellen, John or Mary with anyone besides each other. Gabriel with anybody. (Go figure...) Again, not an exhaustive list by any means. In general, I'm not a fan of romance, so I'd rather not deal with the issue of pairings at all.
Gen or Character studies you'd beta: Dean and Sam, absolutely. John too, though I've got a bit of a slant on his character (basically, Sucky John is great, gruff but loving John is awesome; I'm leery of a John who's all hugs and cuddles). Cas is fine up until the sixth season: I'd be glad to act as a sounding board for S4-or-5 Cas (actually, I'd love it: he's tricky and fun), but anything after that is a mystery to me as of now. Bobby, Bela, Ruby, Ellen, Jo, and Anna are all great.
Gen or Character studies you won't beta: Lisa, Meg, Crowley, anyone who has a major role in S6 or S7.
Beta strengths: Grammar, spelling, voice (esp. Sam and Dean), flow, phrasing, pacing. I've got a strong instinct for characterization, and I can easily pick out issues with Americanisms. I have a lot of experience with ESL writers, so I'm familiar with the challenges of negotiating a language that isn't your first. If you're writing h/c and want someone to let you know when to go easy on the hurt or dial back the comfort a bit, I've got a special fondness for that problem. If it's Biblical knowledge you're wanting (which I see is a tag), I've got you covered.
Beta Weaknesses: Canon. As I've already said, I'm only beginning the sixth season, so anything set after that point is out. Also, I've only watched most episodes once, and I blazed through the series pretty fast, so I may not have the same grounding other folks will. I've got a good memory, so my canon knowledge is okay, but if you're looking for someone who really, *really* knows their stuff, details-wise, I'm probably not that person.
Length of the fics you'd beta: Short stand-alones are my preference (say anywhere up to 5000 words, offhand). I admit freely that I bore pretty easily, and shirk commitment, so when I shy away from your 64K, multi-chapter saga...it's not you, it's me.
Any other info: No first person. Don't ask me, but for whatever reason it just doesn't *work* for me in fanfic.
So....if you've actually made it through that gargantuan mass of text and I haven't managed to turn you off with my narrow preferences and canon deficiencies, I'm impressed and eager to beta for such a hardy soul. I shower you with metaphorical confetti.