The biggest similarity is probably the overhead motion of throwing a baseball and hitting a volleyball. Many volleyball players that are former baseball players often pick up on how to hit a lot faster than other players.
They are both sports where coordination is different than most sports. For example, in most sports you need hand eye coordination for catching, throwing, etc. But in baseball, you also need bat eye coordination. And in volleyball you need arm eye coordination for passing and digging.
You also need a level of power endurance for both sports. Both sports you need to be explosive to be successful.
Also, for both sports you don't need to be in great shape to be reasonably successful.
Play in volleyball is definitely more continuous and you need a much more aerobic endurance then compared to baseball.
There's definitely more interaction between teammates on the volleyball court.
Mistakes are definitely more costly in baseball. For example, an error in baseball could cost a couple runs. In volleyball, if you make a mistake it only costs you a point. Most games in volleyball are played to 25 points, so one point one kill you.