We have no children and I did not think I would enjoy Disney World so was never interested in going. DW convinced me to go several years ago and we did, staying in Fort Wilderness. It was wonderful. We enjoyed it enough that we went back a second time a couple of years later. We booked a package with Disney that included 5 or 6 nights in the Fort along with park hopper passes for us. We did our research and found there are times when the prices are lower along with crowds. For us the week after Thanksgiving was it. The TG crowds are gone, the Christmas crowds are not there yet but the place is decorated for the Holidays. Staying in FW is not cheap but, as noted, their transportation system is tops. We used their buses to take us to the boat that takes you to Magic Kingdom. For all the other parks, we just drove our car and parked in those lots for free. And yes, Epcot is a bit more adult oriented than Magic Kingdom.
Another good time for adults to go is after New Year and before folks start showing up for spring break. However, we found it nearly impossible to get into Fort Wilderness then since the snowbirds started showing up there. We discovered many folks just come to Fort Wilderness and don't visit the other parks.