Depends on what medium and end result the Master is for. Some of the best mixes I've ever heard would never meet radio standards or sound big enough to compete with louder Masters, which is tragic for the industry. But this gets
very subjective. In a perfect world, when a mix is done, that's it--it's done, and the only thing an ME needs to do is merely encode it or transfer it to its destination medium. Classical music should be a great example of this. Hard Rock? Never happens anymore. That was the '70's.
Fortunately, we've reached the top of what is possible, and a real example of carnage left by a bad mastering job was Rush's Vapor Trails (the first pressings). You can audibly hear clipping on many CD players except the newest hardware. It's horrible and fatiguing on the ears. So, we know where our limits are, and what people don't like when it goes too far.
So, I'm happy to say that I want good, loud masters, but I no longer feel the need to keep hunting for the loudest ME I can find.
Bob Ludwig raved to me about his work on the last GnR album, in that he was asked NOT to smash the mix to oblivion for loudness. But in reality, it likely was compressed a little, or limited to get it up to a level where people would still think it was a rocking CD. It's a good Master, for sure.
There's very few (many none) mixes left that really don't need a final touch from a Mastering Engineer. To do that really entails the Mixing Engineer is engaged in modern Mastering practices, most likely. That's not a terrible thing, but it's happening despite what the recommendations are.