Building a Likable character
Something most writers have problems with is the proper way to make your characters likable. If you make your main character who gets nothing but trouble and is generally an asshole, it won't surprise them at all if they suddenly get hurt or killed. The same thing goes for characters that have done really bad things in the past but want to change their ways, if you can't make the audience sympathize with his reasoning, a reader won't stick around and continue reading because they'll most likely think the character didn't deserve all the good things they'll get.
Describing a scene to make it more realistic
Settings are important; they're where are characters interact and build a story from. Simply writing; "They stood in a room." is too plain and doesn't give any insight as to how the scene is going to play out. But when you add details that appeal to the readers senses such as; "They stood paralyzed inside the dark and dank room." it allows the readers to get insight on how the room looks and feels and how the character is reacting to the setting.
Writing motivations for character's actions
A obvious thing when writing a story is how a character goes about reacting to a situation. If a writer writes that character A punches character B, one can assume he's upset. But if you add that the two were jokingly messing around prior to the action, one can assume that he did it without the intent to harm. This also goes for murder stories; why does one character want to kill the other? Are they insane? Do they have a grudge? Are they being payed? People sometimes forget that characters in books are people too and are just as dynamic as real people in our lives. People don't just do something for no reason. Even "because he/she wanted to." is still a reason.
Beta reader Strengths
Plain and simple. I like to read and more often then not, I will read whatever someone asks of me because that person had the courage to ask me to review.