James_Kyle? had an insight for us at Protospiel? 2003. Game mechanics are easier to understand if they all work the same. This is especially applicable to areas such as resource and attributes.

On the other hand, StarCraft? innovatively differenciates itself from previous strategy games by precisely giving up ParallelMechanics - as far as I know, it's the first RTS game where each faction worked in a radically different way, making it in effect three different game systems (though *inside* these systems, there were parallel mechanics).