Centering using counters seems quite a complex way to do "This creature gets +N/+N as long as it's your turn." Is the kind of design of Vithian Dryad worth that complexity?

How is temporariness tracked? How does it interact with e. g. proliferate?

You can put and redistribute the buffs here among the creatures you control however you want. For example, you could have three copies of this in play, which after draw step would be each putting those temporary counters on a fourth creature, giving it essentially +3/+3 UEOT. The counters are there to help with keeping track of it all if you have to distribute say some ten +1/+1 freely among four targets.

The mechanic with most resemblance to this I can think of is exalted, where in the mechanics section where I was even questioning if it was meaningfully different enough but I guess only playtesting would really only reveal that.

That the implementation creates interactions with anything that care about counters is just extra gravy.

The temporariness as I had imagined it would be a designation of the counter itself, akin say whether a creature is monstrous or not. In the case of proliferate, the new counter would not have that state (say if you clone a creature that had become monstrous, the clone wouldn't be monstrous).

This card being centering in blue is weird considering it's marked as a white-green ability here.