The Conquer/Raze combo is nifty if you can pull it off, but it's readily stopped by rivers/fortifications/diverse terrain types (Attention everyone who partied instead of fortifying that first winter, this is why you should fortify your heartland!) And as cerapa pointed out, it's not likely to come up again this game- a1s and GWG are the other two players with high plain density, and a1s has defensive mountains to expand into and the player who could oneshot GWG is Heterodox, so he's more interested in crippling than killing- GWG can start dropping cities in the lakes to defend himself.
The trickier bit to me is that this clearly establishes Plains as the least defensible terrain type; more specifically, they're so weak you can hit a hard target on the other side of open plains as easily as if it were on the border, so they're a huge liability until you fortify them. At least Deserts cost your enemy actions to move across. I think that'll be the crux of how bad this is, whether the cost of abandoning the land is worth the free action you get from it. If you're only intending to cripple your opponent then abandoning the land is an advantage, since your opponent won't be able to Expand back to his original size before Spring (and he'll still have to deal with rebuilding wherever your punch actually landed.)
Again, not a huge concern on this map; there's very few places that have wide open fields. I suppose we'll see how it plays out when two equally matched civs go at it- positioning to keep your enemy out of your plains is now a major strategic concern.