Oh, interesting. I also loved Perdido Street Station and I even more loved The Scar. (Less so Iron Council, although it was disappointing mostly in comparison to the others rather than on its own merits.) Haven't read Un Lun Dun.

And yeah, it's the over-the-top worldbuilding and plot and characters that does it for me: I think in part I loved the way that it felt like he wasn't stingy with the awesome (and awesomely weird) ideas. He could've written a whole trilogy just about the slake moths or the garuda or the Weaver or the handlingers or Lin, and yet they're all in the one book, plus a score of other ideas. I liked that a lot.