Excellent thread this, with some wonderful debate.
I downloaded the bootable AROS last week and ran it. Apart from my mouse not working, I was genuinely stunned at what has been achieved so far.
I have yet to see OS4, and really would like to. If there's anyone in the Yorkshire region of England, give me a shout please :-) .
AROS is a great start, and with a lot of thought I do see it moving forward. In particular, I'd like to see the stuff that has already been mentioned in order to bring it up to speed in the modern OS world - memory protection in particular would be a hot topic for me.
When I get spare time, I will be looking to see how I can help with AROS. At the moment, I can't afford/justify spending out on OS4, and that saddens me :-(