A reboot loop may be caused by all sorts of different things. The N900 may be rebooted by:
[1] user action (N/A in a reboot loop)
[2] a critical dsme-executed system process being killed
[3] a startup script actively deciding to reboot/change runlevel
[4] hardware watchdog
[5] kernel panic (indirectly, the reboot is done by the HW watchdog)

I suppose a "typical" reboot loop is caused by [2] or [3].

[2] may happen if the user has tweaked/modded/deleted something so as to may a daemon crash/exit (e.g. Xorg configuration).

[3] is something I want to investigate: some startup scripts *do* actively decide to reboot, either for good reasons (e.g. after flashing the eMMC) or because they think they're doing something smart (/sbin/preinit, bme, rcS-late, others?)

An example is in /etc/event.d/rcS-late

"If failed to mount /home and system has already been optified - reboot"

Obviously, if getbootstate returns BOOT or SHUTDOWN or an invalid boot state ("Houston, we have a problem") then the N900 will also be rebooted.

This can happen if e.g. the battery is not properly recognized.

I'll try to make a list of things that can go (horribly) wrong while booting.