Projectiles go through non-opaque blocks such as glass, ice, water, leaves, etc.
Summary
So I'm not exactly sure whether this is a bug or an enhancement because the wiki on projectiles explicitly says that you can enable projectiles to not go through non-opaque blocks. I've heard that this issue is already being worked on, but I wanted to put this here just in case it isn't.
Usage
It would allow people to configure blocks that projectiles can and can't go through. Or simply change the current projectile stop at block option to include transparent blocks.
Value
It would allow more versatility to the projectile mechanic.
Priority
I think this is a pretty important suggestion as I have a plugin where people can use water or ice as shields from certain attacks and it would be great to have certain projectiles stop when coming in contact with the water/ice rather than going right through it.
Implementation
As stated before, simply change the stop at block option to include transparent blocks or add some sort of configuration to include additional blocks to not go through.