I have a machine at work, a Dell Optiplex 745, that cannot boot GENERIC NetBSD kernels. There is a problem in one of the uhci/ehci, bge or azalia drivers that causes a lockup at boot time because of a shared interrupt problem. Disabling ehci or azalia from the kernel lets the machine boot. In order to do that, there are two options: either you rebuild your kernel without the offending driver, or you boot into the userconf prompt with -c and, from there, manually disable the driver at each boot. None of the options are quite convincing.
Of course, disabling a faulty driver is not the correct solution, but the workaround is useful on its own. I've just added a userconf command to the boot loader and its configuration file -- /boot and /boot.cfg respectively -- so that the end user can pass random userconf commands to the kernel in an automated way. userconf is a kernel feature that lets you change the parameters of builtin drivers and enable/disable them before the hardware detection routines are run.
With this new feature in the boot loader, you can customize a GENERIC kernel without having to rebuild it! Yes, modules could help here too, but we are not there yet for hardware drivers. Note that OpenBSD has had a similar feature for a while with config -e, but they actually modify the kernel binary.