Well, this guy requires a manabase with a lot of green mana. That's about it.
I don't like comparing cards, but he's like a green Looming Shade that doesn't require you to spend the mana to pump it.
He lets you float the mana instead. And if you don't spend it, he still gets +1/+1 for it later.
So, he's like this:
Mikey, the Elemental Green Looming Shade
Legendary Creature - Elemental

: Put a +1/+1 counter on Mikey.
Remove a +1/+1 counter from Mikey: Add

to your mana pool.
1/1
In a vacuum, this card seems really good. At worst, you can pump him up for free to attack assuming he doesn't get killed, and at best he saves all your excess mana and gets huge.
He'd be good in some green beater deck. But in Vintage, I think he's most interesting in a control deck that's mostly green with a splash of blue. That's easy enough to construct with dual lands anyway. If you can protect him, he'll be about a 4/4 with the option of getting bigger with excess green mana. His ability to float green mana certainly compliments the disruption strategy, since you can use the floated mana to protect him or play card draw, depending.
Of note, without excess mana he sucks at blocking.
I don't think he's broken in Vintage at all. But he is the best three mana creature win condition in a mostly green, disruption heavy deck in Vintage that will have excess green mana during the game, assuming you won't need to block and no other synergies are present to make something like Coatl or Knight of the Reliquary better (which would include card draw, lots of fetch lands, and possibly Force of Will).
Tarmogoyf is two mana, so he's still better in every deck that currently exists. Although this guy could be added in addition to Goyf if there's enough room and green mana.
I don't know how good this guy will be in other formats, since he dies to burn unless you drop him with 3+ extra green mana, at which point, you could play something better for 6+ mana. But if you untap with him in play, he can get out of burn range if you don't make any other plays. So he only has a one turn window against anything playing red assuming you can afford to spend 3+ extra mana into him and keeping it there. Plus he can't block unless you commit mana to him.
He's much better than Looming Shade, and better than Nantuko Shade if you have a ton of excess green mana to leave floating.
Also, if there are any instants that are green and playable this guy instantly becomes better.
So...
He sucks? lol