What are the most common issues you encounter as a beta reader, and how do you address them in your feedback?

I will agree with Annalise on telling instead of showing, it's something I am guilty of in my own writing, so I know just how easy it is to fall into that trap.

Similarly, I've noticed lately a shift toward a lack of description of characters and settings -- I'm getting very basic information about maybe hair color and eye color, and perhaps a wall color or the color of the couch, but very little else. And while I don't particularly have an issue with that as a person with a very active imagination who immediately fills in the details in my mind's eye, I think about readers with varying degrees of aphantasia, who may not "see" anything in their mind when reading. Without at least a little more description, especially non-visual sensory description, these readers may not be able to experience the fully immersive world of reading that others do.

Finally, I've also noticed a lot of head-hopping among those who write in the third person. A tip for those who write in the third person, especially in a multi-POV story: a POV is only considered omniscient if the narrator knows things your characters do not.

For example:

Jessica didn't know it, but her mother was already waiting for her to get home, having heard about her ditching class earlier that day.

If the narrator is always inside your characters' heads, that is third person limited, and you can only be in one person's POV per section or chapter. To switch to a new person's POV, you need a section or chapter break to occur.

For example:

Jessica drove home quickly from her boyfriend's house, already running later than she would have if she'd left school at the regular time. If her mom was home, she'd have to come up with a good excuse, or she would be as good as grounded. 

Telling instead of showing. Too much of "he did this, he did that". 

I like to give examples in my feedback of how they could write it in a more engaging way. 

I also like to highlight the section with too much telling and then ask questions that the author should answer in the improved version. 

Example:

He was so angry. He stormed out of the room. 

My feedback: 

How did he look when he was angry? What expression was on his face? How did his body move or react to the anger he felt? What might other people in the room have seen or felt as a result of this anger? 

As a beta reader, common issues I encounter include pacing problems, underdeveloped characters, confusing plot points, inconsistent tone or voice, excessive exposition, lack of emotional stakes, and repetitive or unnecessary scenes. To address these, I provide specific feedback on where the pacing falters, suggest ways to deepen characters and clarify motivations, flag confusing moments in the plot, and recommend adjustments to tone and character voice for consistency. Additionally, I note any grammar or typo issues, though my main focus is on story and character development.