Nothing.
I have had the Odyssey for about 6 months now and really enjoy driving it. I recently went on my first long trip with it and here are the things I found most annoying with it.
1.The lighting on the instrument cluster does not have a dimmer. It is too bright and distracting for night driving.
2. The road noise is higher than my old Chevy Venture van and it doesn't have as comfortable ride either.
3. Lighting of some of the controls (like on the door) is poor. many of the switches are not lit and thus difficult to use at night.
4. The DVD system controls are very complicated and require fiddling. When you turn down the front speakers while the kids are in the back watching a DVD, the passangers in the middle row can't hear well and the ones in the rear hear it too loud.
5. The most annoying thing about the Odyssey that I have found so far is that you cannot lock the doors with the key fob with any of the doors open. With 2 young children in car seats, I found it very convenient with my old Chevy Venture to lock the car with the doors open so than I could put away my keys, take the kids out and close the door and walk away. With the Odyssey, I have to pull my keys back out, while carrying/holding my kids, to lock the vehicle.
About your comment on the instrument cluster not having a dimmer switch, it does! If you look at the trip button, you can turn it left or right (this dims the cluster brightness).