While I love both DLR and WDW (and I grew up going to DLR), I GREATLY prefer WDW.
As another poster mentioned, DLR doesn't quite have the same feeling of being in the bubble that WDW has. I *really* don't like most of the surrounding "good neighbor" hotels/motels. They feel very low quality to me. I also don't care for walking to the parks past people digging through trash cans (which has happened several times). However, you can easily go to non-Disney restaurants, for example (which I think is a plus for a lot of people). Also, a lot of the non-Disney hotels are cheaper than staying onsite (although with our family size, we couldn't find any "cheap" hotels near DLR - they were 2x what our onsite mod at WDW cost per night).
DL feels much more cramped than MK. The walkways are narrower, attractions are closer together. When I'm at DLR, I really miss the themed/interactive queues that WDW has. A lot of the DLR rides have simple rope lines with nothing to distract you from the wait (and it think seems like more people cut in lines since they can just step over the ropes). On the other hand, DLR is more "walkable" than WDW, and is therefore probably easier on a lot of people.
Some of the rides at DLR are "better" than their WDW counterparts; conversely, some at WDW are superior. (For instance, DLR has the better PotC and Space Mountain, while WDW has the better Splash.)
Overall, they're both amazing vacations. I wouldn't go to California ONLY for DLR, but we often go to Florida solely for WDW. I'd definitely take advantage of some of the DLR-exclusive attractions, like World of Color and Cars Land.