Brava, Ms. Maryn. Brava!
The very best, most honest, most helpful beta reader I've ever had came from my participation in this Community Known as Absolute Write. I credit her fully with my eventual success in being contracted with an agent. When I'm published, both she and Absolute Write will get a tip o' the hat on the Acknowledgements page. ::waves at her beta and tips hat again:: Until then, I beta read for others, paying it forward, and I hope my ongoing investment here will help others as I've been helped.
Maryn is spot on and has put into words what many of us have thought - and done - over the years. I started out betaing for anyone who asked. Soon I had to put some limitations on genre. Finally, I had to put limitations on length, as well, offering to do the first fifty pages only. Most folks thanked me... a few got the feedback and didn't even acknowledge it, let alone say 'thanks'. Those who were grateful were 100% members of this community; the latter were folks who hadn't been around long and didn't stick around long after. People who wanted to be pros vs. people who were looking for confirmation of their perfection? I dunno for sure, but the longer I do this, the more I think so... thus the limitations on my investments. Not only do I look at a members time on AW, I review his/her participation before I commit.
So... what's my point here in this ramble? Oh! Yeah!
Excellent set of guidelines, tips, and definitions, Maryn. I'm voting it be stickied, too.