I also definitely see where the Six Flags comparisons come from, at least regarding guests who are more accustomed to the Magic Kingdom, and it's not a matter of quality so much of scope and presentation.
In Orlando, so much effort is put into the approach to the park's front gate and the reveal of not only the castle, but Main Street itself. Most guests will not have seen the trappings of the everyday world, like gas stations and billboards, for half an hour before they enter.
At Anaheim, there just isn't the same impact because there's no build to the reveal. A guest walking off of Harbor Boulevard is going to turn a corner from all the motels and traffic and immediately be confronted with a smaller version of what she's familiar with at Orlando. If you come from Katella or somewhere to the north, you're also going to be seeing plenty of back-of-the-house as well, especially from the Katella side. At Disneyland, there's less hiding the fact that when you get right down to it, it's just a really, really nice amusement park.
Then there's the fact that everything at Disneyland tends to be a little smaller and a lot closer together than at the Magic Kingdom. To some, this comes off as quaint and historic. To those more familiar with the Magic Kingdom, it's cramped, and there's an inescapable feeling of smallness.
The process is repeated at the end of the day- as soon as you leave the park you're presented with the pickup lane of taxis and Ubercars lined up to leave the park, not a lake.
If you're used to something else, the older, smaller park just doesn't have the same feel, when it comes to scope and overall presentation.
Compare this:
...to this.