We've only been to WDW for a birthday, DD's 6th, my 30th, and upcoming DD's 10th. Her 6th birthday will be a very hard one to top.
First thing in the morning, we got a call from Goofy wishing her happy birthday. We started off the day having breakfast at CRT, where they made a big deal about it. She got a certificate declaring her "Princess for a Day", then Cinderella invited her to be in the parade and told us where to stand so she was sure to find her. After our great breakfast, we got the birthday button from City Hall. Everyone wished her a happy birthday when they saw it. For dinner we went to Chef Mickey's, then off to Epcot for an Illuminations Cruise. We shared a cruise with someone on the DIS and her son brought DD a small gift. Then when we got back to the room, there was a card from Minnie Mouse on the bed. All in all, a great birthday.
It was also my step-dad's 50th birthday. For him....Richard Pettys Driving Experience. He did the Rookie Experience.
I'd say they both had great birthdays.
Now for my birthday at WDW. Who's happy about turning 30?

I met up with some DIS friends a few times. We went to the Dueling Piano place where they 'forced' me and another of our friends to get on stage and do the chicken dance in honor of our birthdays. Ok, I'll admit it, I had a blast.
DD's 10th birthday is going to be a tough one. I just decided on this trip a few days ago. I'm not sure what I'm going to do for her yet. Definately spend the day in MK though. But Shhhhh!!!!!!!! She doesn't even know we're going yet.
I guess I could have just answered that birthdays in WDW are great. No better place to celebrate I'd say.