I finally figured out why Mario & Luigi: Paper Jam is my favorite Mario & Luigi game: nostalgia.
Most people don't like it as much because it doesn't feature any original characters. (Beanish, Hooskis, Shroobs, and new bosses.) But that's exactly why I like it. I can recognize and identify every character in the game without having played it before. (That's not to say I don't like new characters, I'm just glad there's one in the series that features classic Mario characters.)
Just a thought.