Shifting forces:  In order to ensure a reliable valve, high shifting forces are required.  A valve with higher forces will shift through more contamination throughout its life.  How can you determine shifting forces?  Speed.  Higher forces result in faster speeds.

Fast valve = High shifting forces = A more reliable valve

Balance:  In many applications, it is critical that the same result happens each time a valve shifts.  A filling machine must fill bottles to the same levels.  A glue dispensing gun must place a glue dot in the same place every time.  A sorting machine must eject bad parts consistently.  In order to do this, a valve must be repeatable at all pressure levels.  This is especially important to compensate for variations in pressure during normal plant operations.  This attribute is referred to as a