The thing that's most certainly been lost over the years is attraction ideas that don't have a synergy tie-in with a character, movie, or whatever.
To a certain extent, this doesn't matter much. How many people these days actually know that Matterhorn was tied-in to "Third Man on the Mountain?" However there's certainly been a lot more changes to connect characters and recent company offerings to attractions, and less attractions like Jungle Cruise and Autopia which had no commercial tie-in at all.
The loss of Walt is pretty much the reason of this. The reason Disneyland has trains is because Walt liked trains. The reason Disneyland had a saloon stage show was because Walt liked westerns. The reason the Jungle Cruise exists is because Walt liked touring exotic places. The reason for Main Street was because it looked like Walt's hometown. The reason for the Haunted Mansion was because Walt wanted a haunted house, etc. Without Walt's passion to direct new attractions, Disney has been almost exclusively relying on it's entertainment holdings for inspiration in the parks.
People who cry about "Pixarland" and Buzz-Fever need to consider the alternative, though. Do you really want today's Disney executives to build monuments to their interests? I remember lots of talk about how a certain second gate had a boardwalk carnival area representing Michael Eisner's fondness for amusement parks on the pier, and Disney fans raised hell mostly because it was something Walt was specifically NOT interested in.