Do you consider the target of 200 kg per success to be a threshold which determines whether you can affect an object, or merely the mass which you are able to move? If I don't roll well enough to lift a car, can I still lift one end? Because if I can lift half of it, I can probably flip it over, and spin it on its roof, even if I can't get it off the ground.

The spell quite clearly states that the threshold is 1 per 200kg of mass. As thresholds are all or nothing, per RAW you cannot lift the car in any way if you do not meet the threshold.

As a GM, if you clearly stated before casting the spell that your sole intent was to lift part of the car (ie lift the front of the car off the road so that the FWD couldn't propel it) then I would assess a threshold modifier so that you were only trying for half the weight of the car.

If you are just trying to wreck the car, I'd recommend going for Ice Sheet. Yes, the DV has a higher base, but the force necessary is going to be much lower. Or there is always Wreck (Automobile).

The 200kg/net hit is the maximum that the caster could lift. He could choose to pick up a pack of smokes with a spell that had 6 net hits just as easily as he could lift a Harley.

Having extra hits beyond the threshold isn't a problem, but if you are trying to pick up a car which has a threshold of 6 and only get 4 hits, then you have failed the test and didn't lift the car.

Just making sure people knew that. Honest! It wasn't at all because I misread the question. Shut up!

That's where we get into the problem of thresholds being all or nothing. As a GM, I'd let you sustain it if you stated up front that it was your intent. For me, Magic works a lot on intent--if you really don't think you can lift something (ie don't provide a high enough Force to do it) then you won't be able to. OTOH, if the mage intends to lift up part of the car to make it easier for the troll to move, then it may be possible.

Given the teamwork angle, I'd probably run it as a teamwork test with the Mage throwing in Magic+Hits as the effective Str to help the Troll move the car.

You can also run it the other way, use the trolls lifting capacity and strength test to figure out the reduction in threshold for the magic test.

Another thing that came up in my group is that the speed at which the Levitated object moves is Force*Net Hits, so a F1 Levitate doesn't actually move anything. You need one hit to meet the Threshold (up to 200kg), and you don't have anything left over to determine speed.

Let's say an object is a Threshold (3), 600kg thing. The mage casts Levitate at F3, gets the 3 hits to meet the Threshold and now the car is Levitating, but not actually moving anywhere. How far does the "weightless" car go when the troll kicks it?

The troll's kick would have to overcome inertia, but as long as it remains within range of the mage's spell, friction can be removed from the equation.

Shufflepuck with levitated cars is an awesome game for trolls!
It's like Strongbadzone but better!

"You pull straight up to reduce friction, I push laterally" sometimes works well in moving furniture... as long as they're sufficiently coordinated. I can easily see the mage losing "grip" on the car if the troll kicks too suddenly. One teamwork test to overcome Threshold 3 mass, and another if the outcome is tricker than flipping the car.