Slashdot has a quite geeky use base, it might be supposed, that a lot of them would be willing to use a browser plugin to access Altslashdot, if the incentive would be save the community, disable ads etc. Still, a centralised webhosting would be needed, e.g. to attract novices or to support rare browsers.
The idea is, that if a lot of users use a bittorrent--style decentralised hosting, running the centralised web server would be cheap enough to live off ads and donations.
There is some discussion about a relevant technology already. See e.g. Could BitTorrent Be The Distributed Social Network People Have Been Clamoring For?. In fact, a rudimentary technology that would serve a static site exists already, e.g. btsync based web sites.
Surely, a lot od development would be needed. But the upside would be, that Altslashdot would be independent of businesses, dependent on users -- a good thing, as it would serve only the latter.
"Cheap" services like authentication or handling of user data like mod points could be done on the central server -- it is rare, that someone logs in or mods a comment, when compared to reading contents. Also, the former requires only a minimal bandwidth.
While centralising authentication and user data would make things much simpler, a special care should be given to limitting of permissions of the central server owners. For example, it might be an inherent part of the protocol, that:
- important data is mirrored outside the central server; this includes authentication data, but excludes low-importance and often changing data like mod points;
- users can vote on chosing a new central server.