Leaves should act and behave more like spiderweb, although falling or moving through them should not be nearly as slow.

Making leaves having soft collisions would make jungle floors much more interesting, players would have to whack though the think jungle floor bush (maybe with a sword?), instead of climbing on top of the ground bush in order to move.

Players would no longer be able to stand on top of trees and walk from tree top to tree top avoiding the ground. Players would no longer be blocked completely by low hanging trees in dense forests. Nor will players have to tunnel through tree leaves on big trees to get the wood out. Secret entrances would be easier to make and hide.

The only downside I see to this is that hedge mazes would no longer be viable.

p.s. Mojang could even make it so when you move through multiple leaf blocks you are gradually slowed to a halt with each passing block.Realize that the more variation you have, the more code is required to achieve the desired behavior. It would be simpler to make all leaves have a soft collision, and to set the amount of drag even throughout (making it a pain to move through thick bush but doable).