With N900/Maemo you have 3 watchdogs: hardware, external, and software. AFAIK the external is not used anyway.

The hardware watchdog is the one that reboots the device if it's not kicked for more than 30 seconds. The software watchdog is the "lifeguard reset" from DSME. This reboots the device essentially when a DSME-controlled daemon gets killed.

If you disable those two watchdogs your chances of getting into a reboot loop are seriously reduced. The disadvantage is that if something goes really crazy then your device will freeze or drain the battery or explode into a supernova, but at least you'll notice it instead of the usual "huh? did my N900 just reboot and I'll never know why?".

The rest of R&D flags are less important or unknown (to me):
- serial-console: not so useful unless you actually plug/solder a serial cable.
- no-usb-timeout: no idea
- sti-console: no idea
- no-charging: prevents charging. could be useful when testing a battery..
- force-power-key: when turning on the boot reason will be "pwr_key". Some startup scripts read it. I think I have it on for all my N900s, but I'm not sure how important this is.