This is tough! There are too many things, IMHO.
I totally agree about the DP...but I'd add "free" fireworks. Sure, you're paying Disney prices for the event, but they don't charge you extra for the fireworks (unless you don't have tickets to EPCOT!). But the WISHES fireworks are "free!"
Then, I'd say the locations are just amazing! I'm sure there are other venues that are just as gorgeous, but Disney gives you such a variety to choose from. And if you have a reception and a DP or a Welcome Party, you can really mix it up while staying in one place!
The coach, of course!
The Characters
MK Photo Shoot
You have so many options to create an event that is perfect for you and your DF. And regardless of your budget, there's so much magic at Disney already, that it'll be amazing regardless of the size of the event!
We were already committed to the big family affair in January when we decided to do the legal thing via an Escape in June '08, and honestly, afterward I was content! It had been beautiful and magical and I had a blast. It was pure and simple love and Disney magic, which is a perfect combination!
And I totally agree with Betsy. The way things were synched together through DFTW made the whole process that much better!