As a nonbasic land that adds colored mana and has no drawback, Wizard's current design policies require that this ETB tapped.

Wow; ok, now that is a drawback. So it's kinda:

Either ~ ETBs tapped, and makes ; or it's a painland.

Noice! A slight variation on previous painlands; which tend to allow you to make the choice each time, and only provide colourless painlessly - but this one you get locked into your painless choice once you make it.

Being 'basic swamp' rather than just 'swamp' will dissapoint some people - but this is an interesting twist and you're definitely in the right ballpark of "Sometimes I'd put a plains in my deck; sometimes I'd put this in".

I figured this works better as an uncommon comparing it to fetch lands. Also I think it fills a better flavor role. You can still use the land for but at the end of the day it is turning into a swamp.

If it was going to be rare I would remove the basic drawback. I will make an allied cycle next.

­evolving wilds < Decomposing Flatlands < Marsh Flats

That give the whole card, oops. Not a spoiler.

You want triple square brackets [[[]]] or round brackets ((())) :) I fixed it for you.

For the card itself, for some reason I can't help but be reminded of Tainted Field.