This is one of the reasons why 'popist' is a really ill chosen term. I guess 'rockist' and 'anti-rockist' would be slightly better, but...
For me, a rockist is a person who agrees with at least one (but usually both) of the two following statements with regards to aesthetics (we're talking about musical aesthetics, but I see no reason why the structure doesn't apply elsewhere as well):
1. There is something outside of our subjective experience of art that is valid to appeal to in terms of our critical and appreciative practice
2. A particular person's subjective experience is more/less valid than another particular person's subjective experience
Of course, I'm not sure #2 isn't just reducible to #1, so maybe think of them like Kant's different formulations of the Categorical Imperative.
Rockists are usually rockist because, in terms of 2, they think their experience is more valid than someone (everyone?) else's. But we all think that, deep down, or at least that our experience is more valuable (which is a different thing, and this distinction is why of course I don't discount breadth of listening, musical training, etc) than someone else's. The crucial turn the rockist takes is in being insecure enough that they require an objective foundation for this feeling, rather than realizing that since everyone has this feeling (including people the rockist 'knows' are 'wrong'), it isn't much more than our knee jerk feelings that we are special, unique, and so on.
In order for our feeling that we are 'right' and others are 'wrong' about art to be held openly and freely, we have to rationalize a structure that allows people to be right and wrong about art in a way that is, when you get right down to it, indefensible. Maybe your personal taste in music is that danceability is your be all and end all; well then, if you're a rockist, what makes music good for everyone is the extent to which it facilitates dancing. Someone will disagree, because aesthetic experience and that mysterious thing we call "taste" isn't even close to universal; but for the rockist, that person is objectively, demonstrably wrong. They run into so many problems with this for two crucial reasons: The first is that rockists don't agree on their universal standards and the second is that there is no objective way to demonstrate that someone disagreeing with a rockist is wrong. Even about something as basic as melody; not only do some people genuinely and in good faith not enjoy melody as much as other elements of a piece of music, but even among melody lovers there is not consensus about what makes a good melody (as past discussion here suggests).
Now, most people on some level know at least some version of this. And I personally do in everyday speech occasionally say something like "ugh, I hate this band, they're a bad band." But I realise that what I'm really saying is "I don't like this band." I should still stop saying it the other way - it confuses people.
There's a theory much like that in ethics (I can't be bothered to try and look it up - ascriptivism, maybe?) that says ethical statements work in this fashion, that saying "murder is morally wrong" is functionally equivalent to saying "I don't like murder," and it's had all sorts of problems (justifiably so). But that's because one of the whole points of ethics is trying to find a principled answer to the question of what it is about a statement like "murder is morally wrong" that goes above and beyond subjective experience. Maybe they'll find out that there's nothing there, but we feel very deeply (and have come up with a lot of really cogent thinking about the idea that) there's something more at stake in morality than just subjective experience. But with aesthetic appreciation, there has yet to be a really principled, defensible explanation of what's at stake above and beyond me saying "I don't like that song." That's what a rockist would have to do in order for me to stop being so knee-jerk anti-rockist - give a cohesive and, as I said, principled account of what's lying behind there.
Which doesn't mean that I, especially as a critic, don't have things to tackle myself. As Gavin Mueller put it after he stopped writing for us,
So if you've torn down every "objective" standard by which to judge music, and accept that everyone is free to like whatever they want, how the fuck do you write music reviews? It seems to me that the current format of music reviews is indelibly linked to rockism. It continually strikes me as a poor way to discuss music in this "new" critical language. Most critics lazily punt by calling stuff "catchy" or "fun" (like these are descriptors). I, more drastically, stopped writing reviews. If you position yourself as some sort of arbiter of taste, you have to privilege your musical values over other things -- but by "popist" logic, this is (at best) pointless (since everyone has his own opinion) or fascist (since you attempt to "impose" your own opinions on others). And with mp3 blogs and p2p, everyone can listen to the tracks themselves and make their own decisions. So where do reviews go from here? That's what I want to know.
The reason I've kept that quotation, from an old comment on an old Stylus article, is because I think he's quite intelligently and quickly summed up the problems for anti-rockists and anti-rockist criticism. And I do believe there's a principled, cogent account of what criticism is and is for that addresses these. It's what I almost did my thesis on. I don't have the time to tackle it until I've done my actual thesis, of course, but it's interesting to note how well Ingarden's notion of art as intentional object works with this kind of aesthetics.