seems really OP. compare this to modular creatures. I could see this at 3 cost, or maybe rare (probably 2/1). at uncommon probably a 2/1 and only 1 +1/+1 counter. Or add forged or unaligned.
I don't think it's overpowered. The tension that exists within the rules is the drawback here. If you control no other creatures, the counters go on someone else's creature.
But even in the new mirrodin block, artifact creatures don't get 2/2's for 2, especially ones with good abilities. The likelihood is in any limited environment or constructed environment, you will have more creatures, usually a lot of creatures, so the tension is actually very small. And this thing is heads above elder cathar, which is a very strong innistrad limited card, only playable in white, and costs 3 mana.
If you want it to have that proper amount of tension, then I think you can get away with making it a 3/2 for 2, or a 2/1 for 1, but it has to put 2 +1/+1 counters on an opponent's creature when it dies. Then it's actually a serious drawback, for having a very well costed powerful aggro creature for any color.
Was this one my idea? I feel like this one was my idea.