This is such a tough question, and it doesn’t have a clearcut answer. I’ve been to WDW uhm…. 16? times in the past 6 years. I’ve been to DL once, and am going back this September.
As an overall experience, there’s simply no comparison. WDW is hands-down the better vacation destination. It is twice the size of Manhattan. It is four completely separate and distinct theme parks, two full water parks, 20+ hotels, 5 golf courses, two mini golf courses, two different shopping/nightlife districts, and more. Disneyland is two parks, side-by-side, and a small shopping/nightlife district between them.
That being said, I believe that several of the attractions that DL offers that WDW does not are very good to excellent. These include the Indiana Jones & The Temple of the Forbidden Eye, Mr. Toad’s Wild Ride (WDW used to have it, it doesn’t any more), Grizzly River Run, and California Screamin. And I also believe that the DL versions of some of the attractions that are on both costs are better than the WDW versions. Space Mountain is faster and darker at DL. Pirates of the Carribean is about 5 minutes longer with at least 3 extra scenes. Soarin’s queue is better themed. (On the flip side, WDW’s Tower of Terror and Splash Mountain are significantly better than DL’s, and Tough To Be A Bug makes a lot more sense in WDW’s Animal Kingdom than in DL’s California Adventure.
In WDW, you are completely immersed. You are entering a different world, completely cut off from non-Disney. You can’t see anything from the outside world. When you’re at DL, you can see traffic on the highway on some of the taller rides, and as soon as you walk outside the main gate, there’s a Denny’s. I point these out as facts – some would say that’s a plus for WDW, others would say it’s a plus for DL.
WDW is more expensive. A one day ticket is upwards of $80 compared to DL’s one day ticket of ~$65.
Both resorts have outstanding attractions, shows, parades, fireworks, and cast members. I can’t imagine having a bad time at either of them.
A while back I made a comparison of the attractions that are unique to each resort and common to both. It’s a little outdated now, but still relevant: http://spreadsheets.google.com/ccc?key=0AkAI9LTUaudtcGlybGh2anNZR0RPVHY2VGdkcWZWVlE&hl=en
Hope that helps!
(and no, I’ve never been to Paris, Tokyo, or Hong Kong)