You can have world building while also keeping the chains moving. Most quality fantasy authors do this. Hell, Martin did it in the first three books. The world building of the last two books is just meandering, and display Martin's loss of control of his own narrative.
From my perspective, the first three books encompass the War of the Five Kings. We see where the animosity begins, why the major players rise up, and how each of Westeros' leaders fail because of their own mistakes or the backstabbing plots of their families. It's a tight, focused narrative that starts just before the trouble begins so we're always knee deep in intrigue.
Feast and Dance take place after that war has ended. That forward momentum has slowed down as the kingdom settles uneasily under its new rulers. The focus is shifted to the rise and fall of kings to what happened to the rest of the world when the mightiest powers clashed. We see the aftermath of the battles through Brianne's eyes. We get hints of revenge out in Dorne. We see attempts to take over the realm in Meereen. And we see the true threat in the North.
I love this setup. The first three books are fast paced, gripping political thrillers. But then we take a step backward. Treaties and prophecies become more important.
I get that people who read the first three books wanted more straight-ahead action, but this approach makes more sense to me. There needs to be a breath. We need to see the new pieces moved into place before the next major events take place. Yeah, it's disappointing he had to split this into two books, but there is just so much going on that it would be impossible to shrink it to one.
I do think we'll get back to the structure of the first three books in Winds and Dream. Now that's he placed the dominoes it's just a matter of giving them a slight nudge.
