On the bright side, we may have fewer reasons to flash custom ROMs on WP7. What are our current reasons for flashing?
1. We need new OS versions on our devices when OEMs don't provide that. Well, this is supposed to be taken care of by centralized update mechanisms for all devices. WP7 will also support partial updates where you don't have to change everything but rather update certain components. Also, firmware files should be replaceable - otherwise OS updates wouldn't work. We'll be less dependant on HTC or whomever.
2. We need components from other devices (newer versions of Manila etc.). Well, these won't exist anymore.
3. We want light ROMs. WP7 will need things added, not removed, for the most part, and crapware will be very limited.