Here is what I do: I leave comments. Lots of them. As many as I need. I'll correct grammar/spelling as I see it without making a note (although generally in a different color/bold font/etc.), and leave as much of an explanation after paragraphs as I can. This seems strange because—or this is wrong due to—. I am not mean or cruel, but I won't hold your hand and praise you. Beta Readers aren't reviewers; don't expect me to compliment you much.

I have been beta-reading on and off for three years in various fandoms.

Canon, characterization, little random details. If you're talking about food in such and such a country as if it's wrong, I can tell you. If a character said something contrary to what you're now having them say in canon, I can tell you. My strong point is, simply put, anal retentiveness and an ability to research. Characterization is another strong point.

Grammar. Oh, I can spell, and my grammar is generally good, but it's not my specialty. I don't really know every last use and rule regarding colons and semi colons and apostrophes; frankly I don't care all that much, either (although obviously I do set my standards a bit above "barely readable"). I tend to overlook things—random spelling errors, or even that a sentence doubles back on itself or makes little sense when read aloud. The mechanics of writing are not my forte.

I take whatever I can get. I love longer stories, but I have no real preference.

No real preference. I will take on any project no matter how dire! Although I retain the right to be strict if it's just -that- bad.