To me, ASOIAF's strength comes from its characters, dialogue, and the tightness of the plot (well, in the first three books, anyway).
The world-building is about average, in the modern era. Better than the absolute standard, if only for being somewhat richer, but still pretty mundane. Essos, in particular, really suffers. If Essos was less of an Orientalism hodge-podge (honestly the most compelling society there, in my opinion, is Braavos -- that's probably the most unique and interesting culture he has in the entire series), then I think Westeros would look less substandard. I think Westeros is actually a rather good take on the typical medieval Europe setting, if you look at it from the contemporary context (as in, when the first books actually came out). Today, the whole "subverting fantasy tropes" thing has literally become a trope in-and-of-itself. It's so commonplace that it has practically come full circle. This was not at all the case when AGOT came out. There were not a lot of epic fantasies, written on the scale of ASOIAF (nor with the quality of writing), that portrayed the kind of gritty world GRRM did. We only really started to see that boom, I think, in the 2000's.
So then we come to characters, dialogue, and plot. I would never really call his characters truly "realistic". They're almost a dramatized version of a realistic portrayal. I don't know if that makes sense, but yeah. It's like he takes small details from certain characters (generally their flaws) and then amplifies them. In a weird way, he reminds of Quentin Tarantino. Excellent pacing. Lots of twists and turns (no one can say it's predictable). Colorful characters with amusing, gripping dialogue. Deliciously evil villains. It's like the typical "style over substance", except it's so incredibly stylized that there is legitimate substance to it.
Just using Bakker as a comparison, I think GRRM's relatively more skilled in the basic mechanical skills of writing. Obviously Bakker dwarfs him when it comes to world-building (the metaphysics of GRRM's world are virtually non-existent, aside from warging), and I think Bakker is definitely aiming towards a more realistic approach to his characters, as well as a more "literary" approach overall, whereas GRRM clearly is not.
One thing that both Bakker and GRRM do (though in somewhat different ways), which I think is really a mark of someone that has poured a lot of energy into their work, is the sheer of amount of detail; little secrets, hints, "easter eggs", etc. The kind of stuff that the average reader would never even notice. There's a reason why these series draw such an intense amount of feverish speculation and intrigue among fans. It's because the richness of the worlds allow it to happen. Again, they do it in different ways. Bakker relies more on the metaphysics of his world, and the intentional deprivation of information from readers that the characters are otherwise privy to (which I personally find a bit cheap, but alas), whereas GRRM relies on the amount of individual character motivations and the inaccuracy of prophecies (which again, I find a little cheap). The best fantasy would combine all of these. But then that would be a perfect writer, and a perfect writer doesn't exist.
I will also say that I think people tend to hate on ASOIAF a little more nowadays, since the show has come out and it's crazily popular, simply due to the common reaction of, "Hey, this thing's popular, I'll be cool if I don't like it." I'm not saying that to point out anyone specific here...I mean shit, people who have been posting on Westeros.org for over a decade do this exact thing. And I'm not trying to discount anyone's opinion -- if you don't like something, you don't like it. But there's still absolutely a factor of: the more popular something gets, the more vocal a small minority becomes about how shitty said thing is. It's just a natural reaction. It happens with literally anything that becomes popular in mainstream culture.
And yeah, the idea that GRRM is writing this whole series in order to be popular is strikingly ignorant in my opinion, especially when put into the context of the literary climate that the first books originally came out in. It's very clear that GRRM is extremely passionate about his work and, if anything, is willing to sacrifice popularity for his own idea of "getting it right". I mean this is the same guy that made his fans wait ten years...ten years...to find out what happened to the characters that are generally considered the "most popular". He doesn't really seem like the type of dude who gives a fuck what people think of his shit. He's writing the way he thinks he should write it, as he should. And I commend Bakker for doing the same. It's just an unfortunate fact of reality that a lot people happen to find GRRM's style more accessible than Bakker's. And that's fine. But to try and discern some kind of pure qualitative difference between the two, based on that idea alone, is just silly in my opinion.