If I read your question right (i.e. what HW I need to be interested in porting Maemo to other device besides N/eo900), the short list is(in random order):

HW keyboard
FM transmitter
resistive TS (at least as good as the one in N900)
uSD card slot
user changeable battery
video out

...and few more not so important (like hw volume keys etc)

If I don't read it right(so it should be - "what HW requirements has Maemo to be able to boot on a particular piece of HW") - well maemo boots in x86 ScratchBox with xephyr as X server. Skry made hildon-desktop working on arch-linux. etc. So, HW adaptation is not impossible, but me, personally, won't give up on the above (must have HW) stuff just to have 4 cores idling