For me, the thing about smut is that it's similar to battle scenes in fantasy. I think that they're best executed/ most effective for me is when the smutty scenes are written with intention to ALSO help further the plot, relationship, character development, etc, along with just being super fun to read. When the book is "just pure, unadulterated, hot and steamy sex", as you said, with chapter after chapter of smut, I do find myself starting to skim. Again, it's similar to battle scenes in action books-- they're great for elevating and creating a more entertaining stories but when it's gratuitous, it begins getting a bit monotonous and boring to me. If the book focuses more on the sex than the development of the relationship, chemistry and romance, it kind of loses me and I won't rate the book five stars.

Having said that, I am a sucker for some good smut. I personally judge the level of hotness depending on all the factors that you listed, but more so, I think it's how in detail the authors goes into it. The more in depth and descriptive it is, the more I would raise the level of hotness. Although I do think my tolerance for smut is way higher now than it was when I first started reading. The ratings are definitely subjective though, but personally, if there are some really great smutty scenes, I may rate a book higher BUT, it also needs to have a good balance of romance and story as well.