Hate to sound like the old fart in the room, but part of the problem of trying to cater to adults at WDW is that many of the adults at WDW are there to unleash their inner child (or just act childish, whichever the case may be).
There's an album I picked up somewhere called "Date Nite at Disneyland." In fact, I may have picked it up at one of the WDW parks, but it's a cd-repack of a much older vinyl album. It leads me to believe that, Once Upon An Olde Tyme, you'd fine lounge-y entertainment for adults only, where grownups could enjoy come cocktails, listen to a big-gish band play some upbeat numbers as well as a few cheek-to-cheek slow songs. Probably of a more "innocent" time where people wouldn't think anything of either putting kids to sleep in a hotel room and leaving them alone for a few hours, or letting them run around unattended without fear of being abducted. Now, me? I'd love to see a lounge like that in a resort in which I was staying, provided I was on a trip with additional family I'd feel comfortable watching the kids while my wife and I had a grownup night out. But really, are there enough people who really want a night like that anymore? With a big band, and more elegant cocktails? Or is it listening to the same nn-tiss-nn-tiss music your kid are listening to, just without the kids there? Where you can get hammered without getting judged by tiny eyes?
As far as resort themes go, I find the more muted themes of Saratoga Springs, Old Key West, Port Orleans, and the monorail/Epcot resorts to be "adult" enough. They're not no-kids-allowed, but neither do I feel like I'm walking into an over-indulged man-child's man-cave when I walk into them, either.
Lounges with mature singers providing mellow entertainment.
Maybe a comedy club (or comedians who rotate throughout the existing lounges in the resorts, so there's a comic in each resort once or twice a week).
A "gentleman's club" aesthetic (though without a gender bias), much like the sort of club the Adventurer's Club was lampooning. A place where the grownups can not only have some drinks, but perhaps play some cards or chess if anyone has an inkling, read, talk without impressionable ears listening.
Restaurants open later in each resort, even if it's with a more limited menu
And as others have mentioned, the option of in-room spa treatments
And if some of those things can not be done in each and every resort, inexpensive on-site car services that will take people from resort to resort that would still be part of Disney transportation (though transportation you'd pay extra to use) and that wouldn't take you offsite.
But really, it's all a moot point. We're in a "the customer is always right" world, and barring actual bars where you'd legally have to exclude people under the age of 21, any place you'd want to go will have children there, and often they'll be poorly-behaved (usually because they're off their schedules, being park commandos from dawn to dusk, eating poorly, etc) and if you complain, the parent's response will be "hey, I paid $X to be here, we're going to do whatever we want, you don't like it, leave!" And any attempt for there to be something nice for grownups to enjoy will be botched. And that, of course, is assuming all the "grownups" can agree on what constitutes Disney-appropriate adult entertainment. For most people, I think they'd be fine having a place where they can bump and grind, play Wii, and drink with impunity...act like teenagers without worrying about being carded, really.
