I am still waiting (excitedly!) for my first WDW trip, but some of the things I have appreciated about DL while making my plans...
-Proximity of parks to each other. The parks share a plaza. Parkhopping takes about 3 minutes.
-Proximity of hotels. While there's a lot to be said for the all-immersive Disney experience of WDW, there's also a lot to be said for inexpensive hotels on Harbor or the higher priced Disney hotels which are in easy walking distance to the parks. Heck, GCH has it's own gate into DCA. If you want to pay the price (which seems to be between a mod and a deluxe at WDW) there's a really good close option.
-More flexibility. I am choosing to embrace ADRs and FP+ but frankly... it's a little weird for me. That amount of pre-planning isn't what I'm used to and there's something charming about calling and saying, "I want to eat at Goofy's Kitchen around 6" and being offered a half dozen times at 10 minute increments. WDW has way more choices and that will be fun too, but the trade-off at WDW between spontaneity and getting what you want is one I'm unaccustomed to having to make at quite so high a level. In California I have seen FastPasses "sell out"... by 2 or 3 in the afternoon on high volume days. The idea that I *cannot* get a 7DMT FP at the parks day of is a bit boggling for a DL vet.
And of course, for me, DL is close. I can drive. It costs less in time and money and days off from work to go. It can be a weekend excursion rather than the 11 night massive undertaking that WDW needs to be for us.
I am told WDW lacks my favorite Disney food, the hot link corn dog. That's one in DL's column. Don't get me wrong, we're super excited about WDW and BoG and CRT and the million places at Epcot that look so good, but DL has it's own special upsides and I agree that they are well worth coming out to experience at least once if you can.