Currently (5.4.1), liquids combine various features into one: First, liquids have mostly the same visual shape. Also, all liquids require a source and a flowing node. Also, liquids have flowing physics. Finally, a liquid also always means you can swim in it. This means you can move up/down with the Jump and Sneak keys.
All of these features are coupled together and cannot be separated easily. Liquids were basically an all-or-nothing deal. I decided it will be useful to decouple everything into distinct and independent features. With the new changes, every block can now independently define:
- Liquid drawtype (visual appearance)
- Liquid viscosity (how fast the liquid flows)
- Liquid physics yes/no (whether players can "swim" in the block)
- Move resistance (how fast players move through the block)
This opens various neat use cases:
- Cobwebs (without ugly hackery, all cobwebs from games and mods are technically liquids)
- Force fields (you move through slowly)
- Liquid nitrogen (flows and looks like a liquid, but you walk though it like air)
- Liquids that flow fast but can be moved through slowly
- Liquids that flow slowly but can be moved through fast
- "Decorative" liquids that don't do anything
EDIT: Actually, the liquid drawtypes are already decoupled from liquidtype since 5.4.0. How time flies …