Sadly, I suspect there is a "Jethro Factor" in play.
That's very possible, which ties into other comments about things that may have hurt the GF, like being themed, accessable, etc.
If that's the case, then the question of course becomes does the Four Seasons development solve that problem?
I'm not sure that it can. I just can't see people paying for the priviledge of staying in a Four Seasons in WDW unless they are "park people", at least to a certain extent. And you would think that the Jethro Factor wouldn't be that strong in that group. If people are put off by themeing, monorails, and other parts of the typical Disney experience, why on earth will they want to go to WDW at all? A Four Seaons maybe more their type of place, but there are Four Seasons elsewhere that don't have all that nasty Disney stuff going on, like in Las Vegas and Hawaii.
The upshot of all that is it's possible that the Four Seasons crowd and WDW just aren't that compatible. Much like how the family market and Vegas didn't work out well. Honestly, I'm not sure, but I do have my doubts about this being a good idea for that very reason (among others).
Now, if the problem with the GF was more Disney's execution, then that's a different matter. The Four Seasons most definitely knows how to treat that market, but at the same time, if WDW's slice of that market wants both Disney immersion AND Four Seasons level service, this project could still have a tough time.
Some things to look for imho will be how things like transportation, dining, etc. will be handled. Will the Four Seasons have any special privileges that the Disney Deluxes don't have? Will they be part of the same bus system that the "Jethros" are on?
And as others have said, how accessable will the amenities of the Four Seasons be to the "Jethros"?
As for dress codes and such at Four Seasons in other warm weather locations, I looked up the Las Vegas and Maui locations. There's nothing on their websites about attire in the common areas of the resorts, however, they do have dress codes for their restaurants posted.
The Las Vegas location has one restaurant listed as casual, and one as business casual. Of course, what exactly business casual means I'm not sure... Disney and Shula's use that term to mean dress shorts.
The Maui location has three restaurants. One is listed as casual, the other two allow swimsuits with cover-ups for breakfast/lunch, and resort attire for Dinner, which I'm sure means shorts are ok.
Now, Hawaii is about as casual as things go. You can probably count on one hand the number of restaurants in the whole state that don't allow shorts. But still, I can't see a WDW Four Seasons being able to limit visitation through a dress code anymore strict than what there is at other WDW signature dining places. Maybe at one restaurant like V&A's but that's probably it, and that wasn't enough for the GF.