Well, smut manga typically features a lot of sexual imagery and scenarios. Regular manga, on the other hand, can be about anything from superheroes to school life and doesn't focus mainly on sexual themes. The main difference is the emphasis on sexuality in smut manga.
One example could be 'Love Hina'. It has a great story about a young man and his relationships with various female characters in a boarding house. There are some romantic and comical situations that border on the more 'smutty' side at times. The character development and the overall plot of the protagonist trying to find his place and love make it engaging.
The plot itself should be engaging. It can't just be a series of sexual scenes. Take 'Nana' for instance. It has a story about two girls named Nana who are on different paths in life. There are some romantic and passionate relationships in it. The overall plot about their friendship, career aspirations, and love lives holds the story together, and the 'smutty' elements are just part of the bigger picture of the characters' lives.
It depends. If the portrayal of the yandere girlfriend is done in an interesting and nuanced way, it could be appealing. But if it's overly stereotypical or one-dimensional, then no.