It is too crowded that is true.
The other problem is the lack of rides at the other three parks (DHS, AK, Epcot).
THIS.
First, let me say that when we first starting going as a family, I was determined to follow the Guidebook 'rule' that we not spend more than 4 hours in a Park. After about 4 hours, young children just start losing interest. The Guidebook also mentioned that when asked about their favorite Disney experience, most children say, "The swimming pool at the hotel."
Also, to make our vacation a vacation, we make certain that there is a daily balance between Parks, relaxation, and food (typically, just 1 ADR/day unless we plan a character meal as well). As our child gets older, we have been able to add nighttime activities after an early dinner.
Magic Kingdom has enough for us to do. We start in Adventureland and continue clockwise towards FP+ in either/both Frontierland & Fantasyland....and seemingly leave before stopping in at Tomorrowland. Or, we go counter-clockwise and end up missing attractions in Adventureland.
EPCOT is a Park I have come to enjoy, more because my wife enjoys it.

I have little use for the World Showcase, but the stroll with my family is no less a vacation than a sandy beach or wandering the streets of a another tourist destination. Future World could use another fun attraction or two, but we seem to make it work because we are always there during Flower & Garden Show. If I have one complaint about Future World - in many ways, it is starting to look less like the Future every year.
Animal Kingdom is often the least favored Park on DisBoards, but it might be my favorite. Certainly, more attractions are necessary, but we love the shows. Were it not for Lion King (closed for our upcoming stay), Nemo, and Flights of Wonder, we'd never go. My child will be too short for both Primal Whirl & Everest for a few more years, an she is done with Dumbo, er, ah, TriceraTops. We can only hope that by 2017, Avatarland will offer something more than a stroll through Pandora.
Hollywood Studios is our least favored Park. Toy Story is a madhouse that should just be doubled in size. Once again, our child is too short for Rockin' Coaster for a few more years...but just about outgrown Disney Jr. Also, we find the food the least enjoyable here. Fortunately, we stay at Boardwalk and enjoy the food poolside at the Dolphin. But in the end, there is just too little to do here that is fun. Carsland and/or adding more Pixar just seems too obvious to the rest of us.
So after reviewing all I mentioned, I can come to only one conclusion - folks who are disappointed in Disney probably spend more time and/or want to spend more time in the Parks for the pricey admission.
If this was my goal, I'd probably be more empathetic. For better or worse, better in my case, we have found a reasonable balance. Moreover, we found that FP+ makes our trip more enjoyable - no need to rush into the Parks to minimize time we spend on lines. Between FP+ obligations, we simply locate those 'kitschy' things like Country Bears, Carousel of Progress, shopping, etc. in an attempt to follow the KISS method (keep it simple stupid) that makes us want to go back 2X a year. So far, so good.
Of course, in our case, each trip is a new experience for our young child. Maybe she is tall enough to try new things, or we have eliminated things for toddlers and added new attractions. More simply, as we get better accustomed to the Park layouts that seemingly make the Park(s) shrink, we can add some of those things that we had previously passed.
I guess what I'm really getting at - for us, going to Disney World is not about how much can we do in the Parks. Our 'must-do' list is likely shorter than those who (rightly) complain. The Parks are our morning activity for what remains a resort vacation that includes shopping, dining, relaxation, and just spending invaluable time as a family.