Not necessarily a complete list...

Most of the available hardware in the OMAP CPU is proprietary or requires NDAs (DSP, IVA, MBX)
WiFi is mostly closed
Battery and watchdog management is closed
Possibly other stuff

It certainly looks like it's in Nokias interest to fully open the OS sooner rather than later. There's some annoying stuff such as the battery and watchdog management they could open source relatively easily, the other hardware may be beyond their control which is why in future they should use open-source friendly hardware and not the closed source cr@p peddled by TI.