I'm thorough - as in sentence by sentence commentary, suggestions, and in one case rewrites (only after being asked, of course).
I can't bring myself to half-ass my part on something that someone else has poured all this effort into, and its damn time consuming.
So darlings, please don't be offended when I won't beta read your 200,000 word opus - I just don't have the time to do it properly, and wouldn't feel right doing a less than perfect job.
Also, I have been shown pieces that aren't ready for a beta read. I'll say that as well - for some reason or another (be it spelling, grammar, or 'find the story') the first few pages indicate that the draft isn't ready for that kind of scrutiny.
It doesn't mean you suck, it means you have more work to do.
Anyhow, back to the current beta.