Miguel left GNOME before 2.0 shipped, and started working on Mono full-time. If you've ever used GNOME 1.0, it was barely more than a set of ORB-based applications you stacked on top of Enlightenment DR 0.15 (or Sawmill/Sawfish if you weren't cool/used RedHat after Raster left). The panel was awful, the fonts were awful. Themeing (once they moved to gdkpixbuf rather than imlib) was awful. It didn't even have Human Interface Guidelines until Eugenia Loli-Queru basically forced her opinion on the devs.
Sun Microsystems had to dedicate months of programming and useability testing to get the UI to a point that everyone didn't hate it anymore.
Once Miguel gave up the reins, others took GNOME where it is today. This is similar to Lennart Poettering, wherein NetworkManager and PulseAudio became much better/more stable/useable when he left.