Though a lot of author's dislike editing, it's something I thoroughly enjoy. Going back through anyone's work and making it shine is something I consider fulfilling.
That being said, when I beta read, I will mark spelling and grammatical errors, look for inconsistencies in tense or voice, and fact-check when I can. I'll keep a close eye on your plot, setting, characterization, and pace, as well as point out any plot-holes I catch.
I won't sugarcoat, meaning my honesty may come out a little biting to you. I'm not trying to hurt your feelings when I edit.
It may take me some time to finish beta-reading something; usually I edit over the weekends. I will not edit at all during test weeks, vacations, performance weeks, or hell-week (for anyone not familiar with theatre-terms, hell week is the week before opening)
I also refuse to edit too many stories at once - they prevent me from giving each story the attention it needs. Otherwise, though, I'll take pretty much anything and have a fun time with it.