I do both - I beta and I write - and I agree with pretty much everything you've said here, especially about communication between people. I've only really had one bad experience with beta-ing for someone - it was a long fic (70,000 words) and I had told them in advance that it might take me a little while to get through as a result. They responded that it was fine, and then it was posted a week later, without informing me that she'd found another beta. That irked me, because I felt like I'd wasted my time and gone out of my way for someone; that is, like you said, an issue in communication.

The only thing I think I'd add to your advice is for writers. Even if you need a beta for line-editing, if you know you have a major problem with, say, tenses, it might be worthwhile to check over the fic and see if you can clean it up a little, not just send them an e-mail saying "I think some of my tenses are a little wrong." The beta can certainly catch any leftovers, but I always find it much more difficult to take in the whole story and issues with phrasing if I'm constantly trying to catch verb confusion and determine what tense the author might have meant it to be.

Thanks for writing this! You make a lot of good points for both betas and writers.