Actually they used to have international webfares (which were for the FOLLOWING weekend, and were pretty decent). However, due to "peak international spring travel" (for about the last 3 months!), there haven't been any int'l fares offered.

As far as the other suggestions (Fri-Tues flights, upgradeable/BE seats, flights for next weekend), they all make sense. My guess is that for whatever reason Delta isn't that interested in the webfares to really make a big effort in marketing new concepts. I don't know what logic they use in selecting flights, its not the obvious one of choicing empty flights (one I was on go my only DL overbook voucher and bump to BE).
One guess might be that they have Priceline as an outlet to dump superlow priced-tickets that they don't have to push to dump more through webfares.

Jeff