The massive world building/history on display in Tyrion's journey is my favorite kind of fantasy writing. Really excellent stuff. And the Valyrian history is so interesting - I'm sure GRRM will reveal more in the last two books.
I like world building as well, sometimes even more than the actual plot of the book. As a random example, I just picked up On Basilisk Station and definitely liked the sections explaining society better than most of the plot--although the patrol ship in space plotline did have its merits for me. I also am a fan of 'For Want of a Nail" which is written as a history book from a world where England put down the American Revolution. There is no plot there, it is all world building.
I also chewed through a Dance With Dragons in two or three days at most. It only took me that long when because real world forced me to put it down once or twice and because I knew that once I finished it I would have to wait a while before any more of the series came out.
That being said, Martin's world building is now coming at the expense of the plot. I got into this series when I heard it described as "The War of the Roses" with the serial numbers filed off. Then it got better. Then the plot stopped for some reason.
I don't really care about 400 pages of world building when those 400 pages stretch out what should have been five chapters. I was able to forgive AFFC because it was essentially the backstory of the second trilogy, at least in the original plan, for not having a plot. Now that Dragons doesn't have a plot I have grave doubts about this series.
Personally I am looking forward to TWOW and hope that it basically sets off a hard run of two or three books as tighly plotted as the first three to end the series. But I am afraid that world building will again get in the way.