Duplication of effort
If duplication of effort in any area can be kept down to KDE plus GNOME plus one-or-two others, then we are on to a good thing. The thing that must be paid attention to, is that it should be almost accepted that both projects approach things in their own way, but will happily assimilate (with credit) an invention of the other party once it has shown its usefulness (if not before).
KDE/GNOME bashing should be kept POSITIVE!
If, say, the GNOME project appear to be going down a route that the KDE people have explored, but have had problems with, and then rejected (e.g. CORBA messaging), there should -- in fact MUST -- be an open forum between the developers of the two projects. If (in this case) GNOME want to continue, they get to discuss potential problems, and have advance warnings, so that they can learn from 'their' mistakes before they make them.
p.s. don't trumpet 'duplication of effor' as a problem for the sake of it. With the duplication we see between KDE and GNOME, it should become apparent that the level of redundancy in production gives rise to a comparable level of robustness in the end products (since if GNOME figure out how to do something, and KDE have a hard time with it, they should group themselves around GNOME's solution, and vice versa).
But please -- if there's any arguments between the projects, leave those to the developers.
Documents, etc. Not up-to-date multimedia
Information that is useful both now and in a years time should be the main use of such a system. Small-time web pages put up for the sake of demonstrating something are the usual victims of the slashdot effect, and they are the
ones who would gain the most for such a system. A freenet-style system could/would make a valuable COMPLEMENT to the current web. Content more suitable for a freenet system would end up using the freenet system and content which is unsuitable would remain the way it is.
Nevertheless, I don't think that the browser should carry the weight of the logic. Basically, a mini-freenet node program/daemon should do that, and there should be a simple interface added to mozilla in order to connect with it. Given that, you could easily use such a system from mozilla but would neither be forced to use it where it was inappropriate, nor force to use it at all.
That said, a demand driven decentralised information system is something the the internet does not have, but could make good use of. That said, public key authentication needs to be more widespread --- since it would be necessary for many things to be authenticated (e.g. software releases could have a code/version number encrypted via a private key -- the software packages could then be easily authenticated, though this would require a little scrutiny in the design stages...)