For my family (older teens, parents in their 40s), it boils down to attractions. We live on the west coast and much prefer
Disneyland to WDW. Why? One reason is there's so much new (although the original park needs help in Tomorrowland). UO has been introducing new attractions at a furious pace.
To me, the World feels stale and stuck in time, with relatively little added by comparison.
I must have too much free time this morning. I decided to check my impressions against reality. I wanted to look at the most recent two major built-from-the-ground-up attractions at each WDW park. The oldest is Mission Space, so I used 2003 as the starting point for my look at UO. I listed rehabs that changed the ride experience too, such as Test Track 2.0. I did not new list restaurants, meet and greets or shows, nor did I count the faithful reproductions out west of Space Mountain or Big Thunder (despite its new ending effects), or Epcot's Three Caballeros.
I invite you to come to your own conclusions. After looking at this list, I think I've been mistaken about WDW. There are nearly as many new attractions as UO, although UO's are stacked up in the past few years. Neither resort touches Disneyland, which added a full new park and then relaunched it all since 2001.
Now that I think about it, the reasons my family now prefers UO to WDW come down to the types of new attractions, the need to plan as far as 6 months out, and the cost of staying onsite.