A friend of mine just asked me this question. For me, it's several things:
1. It's a place where everything is as it should be. Well...that's probably an exaggeration.

But it's probably the closest thing to it on this earth. And not to start a debate, but IMO that aspect of WDW used to be better than it is today.
2. It brings back happy memories of the only real vacations we took as a family when I was a child.
3. I was brought up with Disney. And my mom passed on her love of Disney. We went to see Disney movies, watched the Mickey Mouse Club (the original, in reruns) after school, Sunday nights we watched the Wonderful World of Disney. So again, it connects me back to my childhood.
4. There's always something new everytime we go. And we haven't even seen it all!
5. It's a great, family friendly, (usually) warm weather destination.
6. The attention to detail is something I appreciate. And I also love the themeing of the various hotels, lands in the MK, countries in WS, continents in AK, etc.
7. Seeing the joy on any child's face when they meet a character or see the castle or ride Dumbo...I could go on and on with this one!