[JoshTriplett]

It's rather good at 4 mana, though that could be balanced in the metagame by making it rare or mythic.

What zero-cost creatures did you have in mind? Those aren't incredibly common. And in any case, a combo that requires 3-4 specific cards is not overpowered; one that only requires 2 specific cards often is.

[tarblog]

Fair enough, but this card doesn't hinge on its combo ability. Imagine it in BW weenie with things like Anafenza, Kin-Tree Spirit.

Even the synergy with Fume Spitter is pretty great. :)

[Ntrails]

You cannot balance a card [in constructed] by making it rarer. You just make it so fewer people can run the deck based around it in a given tournament.

It wouldn't be particularly broken in draft anyway - which is the only meta where rarity does impact balance to some extent.

[lsiebert]

you need two zero cost creatures and a way to sacrifice them for benefits, then just trade off. I can think of infinite mana with four cards.

[JoshTriplett]

Magic is not a game that considers all combos a bug, and there are any number of ways to get to infinite mana (or infinite many other things) with less than four cards.

[jldugger]

Yep, Ashnod's Altar, that card, and any 1-2 cost artifact creatures and you're set.

FWIW, it's about the same cost as Enduring Renewal.