Probably the biggest difference for me is growing up.
Then: I had no idea where we were other than we were at WDW (Just the MK) in Florida and there was nothing else around. (1970s)
Now: I can see it from above via Google Earth, mobile phones work there (which is a good thing but now you're not as remote), and it's less magical and more just a business.
There are so many things I could do a "then and now" on:
The trees in the hub vs expanded hub - no trees.
The highway in the sky! Monorail! vs "We run them so much that we can't fix or clean them"
Pirate changes have already been mentioned. Now pirates are passive creatures from 100s of years back instead of rotten scoundrels, but it makes us all feel better, I suppose.
When I was a kid it seemed more equally theme to both boys and girls. Now it seems to weigh heavily on the girls side with princess stuff everywhere.
Individual shops on Main St. vs the huge Emporium
Epcot was forward-thinking instead of a historical reference into our past.
Tomorrowland was about the exciting world of Tomorrow instead of Pixar/Stitch.
Less trees vs more trees. The park really is prettier, overall, now that things have grown in.
Back then you had fewer (none) empty-non-used buildings (think: Diamond Horseshoe, the Tomorrowland Terrace which I've only seen open once in the last 10 years - sure - you can buy your cupcake/fireworks party there but it used to be a restaurant). The Odyssey over in Epcot comes to mind, too, as does the Life Pavilion.