In seems, from that link and another that of course I can't locate at the moment, that the most significant changes will revolve around what I think has always been one of the weakest points of iWeb really- publishing and dealing with multiple sites and changes.
While renaming domain.sites files or using utilities like the free "iWebSites" program have made it livable to use iWeb for multiple sites, publishing changes to individual sites (rather than all at once with the built-in functionality) has always been a workaround that it seems should have been a built in function. Now that it has been addressed, I think that is a good step forward. Additionally I saw some information that indicated that iWeb will now finally only publish those elements of a site that have actually been changed when re-publishing a site rather than the entire site each time, so that will be a
major timesaver it that indeed is the case.
All in all, those changes alone are worth the upgrade IMO- particularly when you consider how cheap the whole iLife suite is in light of all the apps you get in it for the price- quite a value. When I think of all the dollars I have spent for other company's software upgrades, a $79 upgrade for the whole suite is more than worth it to me!