Iv'e been waiting on this for so long: Fluids placed with a bucket treat other fluids as solid blocks, allowing you to place one on top of the other. This is not true with fluids not placed with a bucket. This way, making artificial ponds is easier.
It will no longer be a pain to make pools, I remember needing to do it the old way it took me hours to even fill in a small pool.
To not make this thread pointless my question is What update are you most looking forward to in 1.9?