As someone who began reading adult books at an early age, I'm no stranger to the idea of reading across the board of genres, regardless of the age group that they are being marketed to. And when I see a well-meaning person who isn't as avid of a reader describe a Young Adult book as being "so good it could have been an Adult," I often give them the benefit of the doubt. People who aren't super interested in publishing are usually not going to be as hyper aware of the aspect that a lower age group for the target audience does not, in fact, equal lower quality literature, and hey--at the end of the day, they are reading as well as taking part in the number one marketing process that is word of mouth.
That being said, I won't lie: the "it reads like an Adult!" compliment isn't quite as easy for me to digest when I see it being used in the midst of the YA community online. From what I can tell, the biggest reasons it is used is to describe either especially sophisticated language, complicated plots and/or subplots, or characters with a surprising amount of maturity and/or intelligence. When, silly me: I thought that the difference between YA and Adult are the ages of the protagonist and the type of conflicts that are plaguing them (in a purely age-based sense, rather than the level or intensity of emotion involved.)