Wowie. That's a heck of a lot better than No Mercy. As mentioned on Stability, it might be better if it were on a creature, because most decks have ways to deal with creatures, and for something that so utterly shuts down so many decks, you need to provide a way for other decks to deal with it.

I find it funny that you can still work around this without the enchantment destruction. Cast Overrun, then attack, and you might still win with your 3/3 tramplers.

Oh, indeed. It does nothing against a graft deck, not much against a White Weeniequip deck, and an elf deck with Timberwatch Elf would still have a chance too. But nonetheless, it is an utter hoser for the majority of aggro decks.

I would have no problem putting this or (((Unyielding Ice))) on a creature. I just happened to be brainstorming enchantments recently.

As for being better than No Mercy... well, it does add to the cost, and a few niche situations, which you've pretty much mentioned, it wouldn't be as good.