Oh wow, the conversation has really taken off...
So in conclusion all you’re really getting here is a cantrip 4/4, which is good enough for limited, but not Vintage.
I think it’s probably good enough in Standard as well.
The card has already put up league and tournament results in Pioneer and Modern. It is unsurprisingly quite good with Uro, Growth Spiral, and fetch lands.
As for Standard, uhhh...
Uro is certainly the most powerful card in the format, but Omnath has undoubtably established itself as beyond "probably good enough".
One particular interaction that @Botvinik and others seems to be missing is the ability can be triggered on the opponent's turn so you don't have to make mana off a fetch if you aren't using the mana. The key synergy that hasn't been explored in Vintage or Legacy is with Sylvan Library where you turn the 7 life from a fetch into 2-3 cards a turn. This card has the making of a control finisher. Along those lines, it has a lot of value pitching to Forces in those control decks.
Edit: If people want comparisons, is this card that different from a massively power crept Bloodbraid Elf? It draws a card when it comes into play and gives you the mana to cast it if it's a spell. There are differences of course, but the value and mana generation are similar. You also have additional utility in the life gain and the damage.