I have given and received the critique that characters are confusing.
Sometimes their names are too similar. Sometimes they don't have their own feel. In one of my stories, I thought the character names were quite distinct. They looked very different on the page, but said aloud they sounded similar. After much internal struggle, I changed them.
In another story of mine two characters only shared the first letter, but people confused them. One was a very minor character and one was an important character who showed up every few chapters. How could they confuse them? Sheesh. One was a cop and one was a doctor. Isn't that enough?
Obviously not. There wasn't enough to hold an image or a feel or a lingering impression. I put in a description the first time the character arrived and left it at that. But describing his suit and the color of his eyes isn't enough to give the reader a feel for him. I need to think about characters that stay with me.
Jim Butcher's characters work for me. In the Dresden books his protagonist's sidekick is a petite blonde woman, with blue eyes and a button nose, who has multiple black belts and is a sharpshooter. I have a very clear picture of her. Dresden's reactions to her very often involve her looks. He thinks she's cute and knows she'll trounce him six ways to Sunday if he ever said so. But he thinks about it. And that reinforces the reader's image.
J.K. Rowling repeatedly mentions Snape's greasy hair. Robert Jordan has a character that pulls on her long braid when she's angry. Physical reminders of who is acting or speaking. I can see how it is done well. I just need to figure out how to do it myself.