Its a new chipset with an immature BIOS. I think that accounts for about 90% of the issues. It introduces unprecedented end-user control over BIOS options that give total control over system settings. I'm guessing there are still bugs to be worked out considering its a new chipset with considerably more options.
It also introduces a lot of new features that haven't required troubleshooting in the past, such as dual-DDR and dimm compatibility and PCI/AGP locks. Also, the flexibility and support it provides to the entire Athlon Socket-A line (including the just released Barton) is pretty amazing as well, but will also compound any problems. That's not even getting into the amazing integrated capabilities of the nForce2 boards/chipset.
Many of the problems people run into are not b/c the board is unstable or problematic at default, its when you throw in the myriad other factors the board has to juggle when running out-of-spec. As with any new product, there's going to be growing pains, which many are willing to cope with for increased performance.
I think there is plenty of praise for the nForce2, which also leads to more problems. Experienced builders who can resolve most snafus by themselves will adopt early and recommend the board to those "wait-and-see novice types". Then those people go out and buy the board b/c they see some silly benchmarks and figure its plug-n-play like a Nintendo. Doesn't quite work like that; hence the majority of the problems you see here. As with anything that supports legacy devices, there is compatibility issues, but I've rarely seen any "show-stoppers" that prevent the boards from running at default.
Chiz