D2O has different bond angles relative to H2O, basically, and it forms a crystalline structure less dense than liquid H2O.
D2O also doesn't hydrogen bond as tightly. It also is toxic (in high doses) because of the secondary isotope effect which I won't bore you with.
If regular water was dense than ice, we probably would not have life on the planet. Oceans would freeze from the bottom up and kill everything in it. The water deep in the ocean gets to 4°C because that is the densest point for H2O.