[I] Intentionally skipped flash.

(Double Post Deleted.)

Seems extremely cheap. Just putting a counter on a single creature would cost as much on an uncommon and these keywords are easy to come by if any is enough. Maybe if it was "each creature with the chosen keyword that hasn't a +1/+1 counter on it".

This also seems like it shouldn't be instant speed at common.

So, I thought that was a rare slot. Whoops!

This is still a combat trick on a land. It's not a matter of making the land worse and piling drawbacks on it (ETBT and producing just colorless mana?), but the sheer fact that lands are so easily overlooked as sources of abilities that makes me advise against this.

Adding that this is a repeatable combat trick as opposed to the cycle of lands that sac for +1/+1 counters is also a problem.

And this has no shields down moment with regards to other mana sources. As long as you don't need to tap out entirely to your very last generic mana source this combat trick can stay online and mess with your opponent's combat math.

As a common you might have two of these on the board.