Might be necroposting, but here's what I consider as a beta reader:

Content, continuity, and Ease of understanding

Typos, grammar, etc are for proofreaders and editors. I have no desire or willingness to proofread your book. I read through, not as a professional, but as a reader. I tell you what we, your audience, will think (yes, I am presumptuous enough to think I speak for everyone). If you plot takes jagged, hard to follow twists, I consider it my job to show you these, and offer solutions to "smooth the edges" so to speak. If you say it's raining one moment, then your characters spend the day out in the sun, I consider it my job to point it out.

I do it because it's fun. Also, I get to read books so early. It's great. I do, however, have a simple set of 'demands'. It's standard these days to acknowledge beta readers along with everyone else when you publish. In addition, I lay claim to a signed copy of the finished product, as a memento in my personal collection.