The best betas are willing to work with you as an author and as a person. Me, I love playing sounding board to authors I like and respect, regardless of whether or not I'm editing their works. Giving ideas and plot twists to a writer, for me, is fun. I get too many of them for just one person to use, anyway, and sometimes it's a blast to have two writers take the same idea and write their own stories.
As for long chapters, well, one lady I beta for (when I have time, which, lately, I don't...) has been known to send me 10,000-20,000 word chapters. When I had daily internet access, my turnaround time for her was astonishing. I've been an avid reader since I first learned how to read...and have been known to read 800-1,000+ page books in 24 hours or less, without skimming anything. Out loud, of course, I'm not so quick, but I don't miss as much, either, and so when I'm editing anything, be it my own work or another's, I do a once-over silent and a once-over aloud. And when it's posted, I reread it again a time or three, and if it's another's work, I will add in my review that, "I missed this when I edited it. I'm sorry."
Really, beta work is give-and-take, and the best author-beta relationships are friendships with equality--and forgiveness and acceptance when things go wrong. After all, Murphy is everywhere....
As an author, there are a number of things I look for in people who help me with my stories. For example, Raymy will tell me if my editing needs work, but she also tells me what is and isn't having an impact on her and what she's looking forward to seeing in the future. Fei tells me what gets to her, what she's looking forward to, and, even though she's not good at editing English, lets me know if I've confused her with a lack of explanation or sudden missing characters that were there just a moment ago. Clara and Snow tell me how well I'm doing with characterization and world building (futuristic Musketeer-ish Gundam Wing AU is challenging, and I love it, though it takes so long to get through the scenes because it's all stuck in my head and still rather jumbled together), as well as what they're looking forward to seeing. All of these wonderful ladies have also played sounding board for me in one way or another, and when I needed help through rough patches or a handy stick of dynamite for an obstreperous wall or three, they have been there and offered ideas and support, and occasionally some literary nitroglycerin. Ain't friends wonderful?
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Kindly ignore that last bit of atrocious grammar. Or laugh at it, if you'd prefer, I suppose. Sometimes my accent bleeds through, thankfully not as bad as my dad's. His would be completely incomprehensible to those who weren't either fluent in southern drawl or familiar with our family. *sigh* It's a wonder I don't need a beta for the accent alone....