What would be different about a heavy-water ocean world? The water on Earth is primarily made of $\ce{H2O}$. I'm wondering about the conditions in a hypothetical $\ce{D2O}$ ocean world. 

Would it be stable over geologic timescales (~$\pu{0.1–2 Ga}$)? What would be expected to be different, if anything? Would it affect the chemistry of its atmosphere, or appreciably change the incident radiation from its host star?
For one thing, it would be hugely more unlikely to exist.  We are more likely to have worlds where there is a little more deuterium in the hydrogen, not 100%.

Most of the hydrogen in our Universe is protium (hydrogen-1) because the vast majority of unfused nuclear particles in stars are protons.  Neutrons must either fuse with something or they decay to make protons with a half-life of ten minutes.  Deuterium and tritium are formed initially when the protons are fused, but they are just intermediate towards the formation of more stable helium-4 nuclei (alpha particles) and so do not accumulate to large concentrations.

If there were a heavy water world, life would have to adapt to it.  Prolonged consumption of pure heavy water is deadly to life on Earth.