When first built, they had strong themes, you were immersed in whatever message they wanted to convey.
New York-New York's casino floor was like a stroll through the neighborhoods. There was China Town, a statue that was a cross between Marilyn Monroe with her skirt blowing up and the Statue of Liberty, delicatessens, and lots of fake trees.

The Luxor, shaped like a pyramid, had an indoors canal with rides on Egyptian rafts, and a tour of the inside of an ancient pyramid.

The MGM Grand had a movie theme with an emphasis on The Wizard of Oz and the MGM lion.

There was that time when Las Vegas was trying to be a family-friendly destination. There were a lot of rides like roller coasters inside the casinos, consistent with their theme. Some of these are still there: New York-New York still has its roller coaster, the Venetian still has the gondola rides through the fancy shopping mall, the Mirage still has the volcano, and TI (formerly Treasure Island) has the nightly pirate show.
The Paris and Caesars had big outdoor fountains that made us feel like we were tourists in Europe.

Sadly, restaurants and night clubs are replacing these attractions, and the casino themes are evolving to a generic sort of modern blah. As much as they can without completing gutting the casino floors, they are removing the cute theme stuff. Consistent with this is the trend to rename the hotels to sound more modern.
For example:
Aladdin: Planet Hollywood
Las Vegas Hilton: LVH (stands for Las Vegas Hotel)
Treasure Island: TI
Fitzgeralds: The D
Imperial Palace: The Quad Resort
Sahara: SLS Las Vegas
Newer properties no longer keep the desert or exotic location spirit. Here are some of the most recently built hotels:
Aria, Vdara, Cosmopolitan, Encore
Not much character or sense of place there. They don't conjure a sense of adventure or destination in my mind.
I wonder how long it will be before they realize a need to be distinctive and go the other direction.
Here's a list of all the current and past Las Vegas hotels with former names.