We have taken our kids to Disney starting around 5 months old. Honestly, they were the beginning of the best trips ever.

Our first went at 5 months, for his first birthday, at 14 months, at 2.5 (twice), and at 3.5. Our second went at 7 months, 9 months, and at 18 months. Our third is ready for her first trip at 15 months this November.
We had gone PLENTY as a couple, and we'd seen it all. Yes, trips with young children are different, but they are no less fun. If you go with the expectation that you'll move more slowly, you won't be able to weave through crowds as easily, you'll need a diaper/food break when you weren't planning it, etc. you'll be well prepared.
As PPs mentioned, rider swap is your friend for seeing attractions with a height requirement. We, however, have usually skipped these attractions unless we were with extended family. We aren't interested in riding alone, and we've ridden Soarin' dozens of times, so skipping it wasn't a big deal. We made an exception when 7DMT first opened, but that was the only time. The new excitement in our house is that our oldest has FINALLY reached 40 inches. He is so excited for his first trip on Splash, Soarin' 7DMT (38"), etc. He's bringing yet another level of enthusiasm to the trip.
When we had young kids, we did everything without a height requirement. Both loved Small World, Pirates, Pooh, the boat ride in Mexico. Our oldest even loved shows like Beauty and the Beast and Finding Nemo the Musical. The colors and music are very stimulating.
I don't know if you'll still be nursing, but some attractions are great for that - I loved Spaceship Earth, the TTA, Carousel of Progress, Hall of Presidents. If the lines aren't long, I was often allowed to stay on for a second loop if the baby fell asleep or wasn't done eating.
Know your child. That goes a long way in keeping everyone happy. Can he/she sleep in a stroller or do you need the room? Be prepared for that. Does he/she need snacks/meals at a certain time - be prepared for that. Does he/she mind loud noises? Be prepared for that. Like characters or afraid? Be prepared (although we had to test it with each). Need to be in bed at a certain time at night? Be prepared.
Every child and every family are different. Having kids didn't keep me from enjoying life at home, and that is true for Disney World too. I knew what each would need and was ready to do what was best for the kid. And we had a blast!
As long as you are mentally prepared, there won't be unmet expectations. My last piece of advice - take LOTS of pictures and video. We've made a video of almost every trip. It is so fun to look back and see the developmental changes and remember the joy of each trip. The links are in my signature so you can look if you'd like. HAVE FUN!!!!!