Don't take me for a critic by answering the question because I love going to Walt Disney World, but here are my fair minded observations:
It's way more expensive.
The new attractions, when they open, no longer seem to have that "wow" factor that once seemed to exclusive to Disney parks. In the past 20 years, some attractions have opened at Universal Studios parks that have turned some heads. Somewhere along the line, Disney let the competition catch up or at least close the gap in new attractions building.
Also, too many attractions and entertainment have closed or been canceled with nothing to replace them. An entire pavilion at Epcot - Wonders of Life - was closed with no replacement. Attractions have closed at the Disney Hollywood Studios park way before any demolition has been scheduled to occur - example being the Back Lot Tour. The Yeti breaks at Animal Kingdom and . . . it doesn't get fixed. The Magic Kingdom has been criticized for any number of reasons from being too crowded with not enough new rides opening or for taking away benches.
I agree with your belief that Walt Disney World is still a great place to vacation with or without kids. It is just that when we imagine what could be in 2016 after 45 years of operations, many would agree it could have done better in the past decade than it has done.