As I mentioned, my experience with beta reading is limited, but I still like to think that I know what qualities to look for in a person who is going to beta read my work and what qualities I believe you, as a beta reader, should exhibit when evaluating the manuscripts of others:
Don't worry about hurt feelings: Your most important job as a beta writer is to be honest. It's not doing the author of the book you're reading any kind of service for you to sugarcoat any criticisms you may have. As writers, one of the first things we have to develop is a thick skin. Harsh words come with the territory. The fact is, not everyone is going to LOVE your book (shocking, I know). As a beta reader, you need to point out all the errors in the plot, the predictable dialogue, the errors in grammar, and--worst case scenario--whether a major re-write is going to be needed in order to salvage the book. You're there to give your honest opinion; that's why the author asked you to beta read for them in the first place. If they're a reasonable person, they'll be able to take the verdict they receive and better their work. If they're not, then they were never ready to handle their work being beta read in the first place.
Never beta read the works of your friends/family: There is no way you can be 100% truthful and unbiased with those you care about. Think of it as the crappy children's art syndrome. Okay, perhaps that's a little harsh, but for those of you with kids, you may know what I'm talking about. When your child brings you that "puppy" they drew that looks more like an atomic bomb exploded in a room full of Smurfs, your reaction isn't, "What the hell is this crap?" It's more like, "Oh my gosh, sweety. This is so beautiful!" And you mean it, for the most part. When you read the works of those you care about, like it or not, there are blinders that naturally appear. What one may perceive as awful, another may perceive as needing only a slight tweaking. Even if those blinders never show up, there is still the thought that you are going to have to see these people on a regular basis and you don't want your words to have completely shattered their dreams (making for a very awkward Thanksgiving). Read the works of close friends and family for fun, but never go beyond that. Giving someone false feedback is never helpful.
Break out your fine-toothed comb: Next to being honest, a beta reader needs to be thorough. Read each page, digest every sentence. Don't just look for those errors that stand out, look for those errors that are camouflaged in the midst of otherwise flawless, beautiful writing. Be watchful of small glitches in detail. Did a character leave his or her coat at home and it has now magically appeared while said character is in the midst of a 600 mile road trip? Are the characters being true to the original picture the author painted? Is the story flowing like a babbling brook or is it slowly trudging uphill in a snowstorm? After reading their book a hundred times over, the author tends to become blind to the obvious; their minds mentally correct the mistakes. Beta readers provide new eyes and have the capability of spotting those errors in detail missed by the authors after their 101st read-through.
When in doubt about a grammar/spelling error refer to sources: Break out the dictionaries, thesauruses, and various other books on the proper usage of commas, semi-colons and em-dashes. None of us are experts nor do we always avoid making grammatical errors. If you don't know whether something is wrong, refer to the sources. If you still don't know afterwards, then throw a suggestion in anyway. At least this will give the author something to think about while simultaneously quieting the nagging monkey on your back.
Avoid discouragement: As much as you need to be honest with your criticisms, you should never tell an author to give up on their dreams. All writing is subjective. There are published authors out there whose books I prefer not to read as their writing style doesn't do anything for me. But, they're published authors and they have a fan base that I can only dream of. Give the author your honest opinion, point out what doesn't work and what you didn't care for. However, never under any circumstances tell them to quit what they love nor discourage them from writing more in the future. Encourage re-writes and offer to read them once they've completed them. Just because their writing doesn't work for you doesn't mean it won't for others who read it.
Your Preference v. Their Writing: Let's face it, we all have our different tastes and there are just certain books you see on the bookshelf that, although they may be well-written, simply just don't appeal to you because you're not into love triangles, ghosts, robots, cowboys or talking rodents. Hence the existence of genres. Chances are, unless you stick to beta reading exclusively from your own genre, you're going to be asked to critique a book containing subject matter that appeals very little to you, if at all. This is where you need to put your own preferences aside without allowing them to bias your opinion and focus on the writing itself. Sure, you may rather endure Chinese water torture than read a shoot-em-up Western, but you can't let that stop you from focusing on sentence structure, plot lines, grammar, character development, dialogue and all the other jazz that comprises truly great writing.
Timeliness and Follow-Through: When you agree to take on somebody else's work, you're essentially agreeing to make it a priority. Granted, there are those unforeseen events in life that can act as a set back to your duties as a beta writer, but you should still keep on top of things by shooting the author a quick e-mail letting them know where you're at in the book, your thoughts thus far, and a reasonable time frame for completion. Yes, being thorough takes time, but that doesn't make it acceptable to begin another writer's work when the snow is flying only to finally complete it while you're sitting on your back porch in your bikini sipping a mojito. When you agree to take on someone else's work, you agree to make it as important to you as it is to them and to honor the fact that they have their own deadlines and expectations of when they'd like to see their work completed.
What are your thoughts? Have you ever beta read for someone else? Ever had a bad experience with the beta reading process?