Protagonists, (they are the most common in harem anime, go take a look at Dangan Ronpa's and Rosario + Vampire's protagonists as examples. This trope is also HORRIBLY prevalent in video games), that have literally NO personality, and often have a very bleh design. They also are not allowed to actually impact the story/characters around them directly yet they somehow manage to do so by doing very little. You are not allowed to know much about them because you are supposed to project yourselves onto them, it's a way to pander your show to as broad an audience as possible, and thus must have no personality traits or quirks for fear of someone saying. "I don't do that! I can't relate to this anymore! This show is stupid and I'm not watching it!" I don't know if this is a japanese thing but I DESPISE it. If I want to figuratively step into a character's shoes, I want it to be a character that's interesting. I'm a boring guy, I don't want to pretend to be a boring guy, I want to pretend to be the guy that does the cool things.
In video games I use the examples of Legend of Zelda and Metal Gear Rising. While I do like Zelda games, for a very long time I personally would like to see Link become an actual character. He doesn't talk because he's mute, he doesn't talk because YOU are supposed to be link and if he talked then you can't be that character anymore. While at least in games the blank slate protagonist is allowed to do things but you still never know anything about the characters themselves. Who is Link? What does he do? What does he like? What does he Dislike?
Compare this to Metal Gear Rising, Raiden is a character, he's not me he's him. Raiden has goals, likes, dislikes and a backstory. I may not have been a child soldier and eventually become a Cyborg PMC contractor but when I pop Rising in I slip right into Raiden's metal boots. Suddely it's all "YEEEEAAAH! I'M FIGHTIN DA METAL GEAR! LOOK AT HOW COOL IT IS!"
Now this could just be me, but that's how I feel about it.