There's nothing wrong when you get or give a compliment on a story or concept you are working on or reading, Rozel. "Hey, I really like your story, the plot is really unique," or "I really like your character, she's so sexy and evil," and "Just wanted to say thank you for writing such a good story". All of those are perfectly acceptable in my book, and I welcome those types of PM out of the blue. In fact, I love it, because I know there's at least one other person out there, other than myself and my partner(s) who are enjoying the idea. Whether people share my outlook is up to them, but I don't see the harm. At worst, they simply won't respond.
...however, when it turns to "Hey, I think your writing is so sexy, do you want to do this idea?" or "Hey, when you have free time, hit me up for a roleplay,", then, in my opinion, it's unacceptable. You're pushing your way into their space with little to no reason, and it's creating the illusion that the only reason why you want to play with them is because they are good, and that they have standards to live up to. Unless you have a structured idea that you wanted to run by them, then it's fine, but I've encountered some people who simply message me "Your writing is so hot, we have to do a roleplay. It doesn't matter what, just something, right now." People also try to use this to get their foot in the door, kind of like a pick up line, if you will.
Furthermore, players can't push boundaries inside a PM, or be too crude in their compliments. I've heard stories about people telling other people the things they are doing while reading their stories, and it's downright disgusting, not to mention inappropriate. Just keep it to simple compliments, and that way, no one gets hurt.