This is probably the first thing that really hooked me - we've all seen slow-motion mechanics in games - first-person shooters especially - but Cobalt is the first time I've seen it implemented well in a platformer game. There's something endearing about doing a flying roll through a slow-motion field of bullets, spraying return fire in a wheel of lead-laden destruction. Still, it is less of a gimmick and more of a survival mechanism; it's fair to assume that an awesome robot would have killer reflexes and enhanced perceptions, and the enemies one faces often have the same - it's a leveling of the playing field.
It also makes for killer videos.
One thing that ties well into the whole "slow-motion" system is the mechanics of combat. Not just the rolling, jumping, flying aspects of it, but the raw physics - the ability to pinball up the sides of walls, carefully aiming shots mid-roll cannot be underestimated. In many ways it is a skill that requires mastering, since the environment is just as crucial as one's reflexes. In this case, "environment" includes your enemies (or friends), since a well-timed body-ricochet off of an enemy can mean the difference between an awesome kill and a spectacular death.
This only gets better with other players involved - the mix of rage and laughs is an addictive one.
What are some of your best "wow" moments in the game? A particularly good kill, an awesome camping spot, anything at all - tell us about it!