Wait... this is way too good.

Gleep! Yes, it is. You could perhaps have a rare with this ability costing perhaps 4. But no way on a common.

What if it was just "Whenever you tap a creature for mana?" Then perhaps it could be uncommon.

Is this more acceptable?

I don't know what the team was originally responding to, but I was going to suggest a card that looks very much like this the way it is now. This seems like the simplest execution of the ability, so I would like to see it at common, power level arguments of this particular card aside.

I think the original version was "Whenever you tap a land for mana, ~ gets +1/+1 UEOT."

Either variation is quite nifty. Potentially very strong, but only potentially; and rather better as an attacker than a blocker, as you have to save your mana for the opponent's turn to block on her. Still remains useful even without a spell to spend the mana on. So I think I like it.

Ironically, I think this version ("tap a creature for mana") is less suitable for common, because many draft decks won't have any mana creatures in. So maybe I overreacted before, and the other version is actually more suitable for common? Costing about 5 to cast.

Oh. Heh. I didn't even see the fact that it asked for creatures. No, I personally think this should be land and that we should find numbers that fit. If we like "triggers when a land is tapped" then we have to like this card.

The numbers are up to other people. If this is too much of a pain, I'd suggest bumping the CMC and the p/t at the same time. But I want this card to be perceived as playable in draft.

I changed it to creatures because I thought green might have even more mana-producing creatures than normal, to go with the mana theme. That's where Dryad Collective comes from.
EDIT: The original version, by the way, was "whenever you tap a creature or a land for mana."

I don't know if I missed the discussion somewhere and now can't find it, but whatever happened to the "plants care about mana" mini-theme? Did it morph into caring about tapped lands, as on Rootsnapping Kudzu and Heavy Plant?