Not a very spicy card for the final preview, in terms of what it does. I can see why they chose it, though. I mean, we realistically already knew there were a lot of "tribal" cards and "race matters on non-creatures now too", but they didn't actually show us anything until now. And the new "clash" keyword with all it's appropriate "oohs" and "ahhs" (not to mention the "uggs").
I think the problem I have with a lot of this isn't the cards themselves, but how they go on for two or three paragraphs, not explaining how the mechanic came to be, but rationalizing that "no really, this mechanic is cool, we promise..." It's like they expect everyone to hate it right from the beginning.
Personally, without the clash effect, how bad is this card? Double Flash Foliage but without the draw or the timing restriction. Decent, but nothing to shout about. Add the clash ability and you might kill two of their biggest attacking creatures. How is that not swingy again?
The only way I see a card like this getting played in (high-level) constructed is if "race matters" decks are competative and two more elves is really good, or if there are ways to play it other than intended. For example, Cursed Scroll was supposed to be a bluffing card. Okay, that was more obvious than this (Brainstorm, anyone?), but I'm not discounting the possibility.
Good enough card in limited...If removal is tough to get, I'd draft it early. If removal's not a problem though, this isn't consistent enough and doesn't have a good enough to play without relying on the clash.
Personally, I don't like "random" effects like clash. I think they play outside the game and trying to encorporate them always feels awkward to me. Better than a coin-flip, but not by much... Now, if they are doing a lot of "clash" to replace flipping coins, then maybe this is good...