I don't think the danger with
marsh casualties would be the power level of the card per se, but rather the overall number of mass removal spells that the designers want to allow.
And the real problem is actually
cultivate: that card makes it so easy to just put all the mass removal in the same deck, regardless of color. So I think it's good that we only get a total of 4.
I actually think that the biggest design flaw in the cardpool was the inclusion of
anger of the gods as opposed to something like
slagstorm /
flamebreak, since the exiling effect makes it so much harder to build a deck that can fight efficiently against removal, even though at first sight there are cards in the card pool that would do just that:
vengevine and
bloodghast - which just die to anger.
They even included the 7/2 plant guy that comes back from the graveyard as if they were really trying to make anger look as good as possible; and it sure looks awesome in this game.
edit: let me add a creatureless grixis deck that I just made (it's an upgrade to the Izzet version that I posted some time ago):
This version splashes black specifically for
tribute to hunger, which may seem a bad idea since the card is not great in this environment full of satyr wayfinders and tokens.
However this deck actually has to kill (or counter) all those crappy 1/1 creatures since it doesn't have the option of making them irrelevant with a blocker. Which is exactly the situation where tribute is bad: when the opponent has a creature in play that is completely irrelevant, just waiting there in case a chump-blocker is needed. But this deck is willing to shock soldier tokens and counter elvish visionary to stay alive (or just wipe them all away with anger) so that's not really a problem.
On the other hand the lifegain is very important against
banefire, and having a hard removal spell in hand makes it much safer to tap out to play a wand.