It's genius, noone thought about exploring this era this much before. It was a war that started being fought with swords, muskets and horses, during when warmachines such as tanks and zepellins appeared, and the rules of war changed a lot.
Taking this concept and using it with some distortions of reality is a really good idea, specially in the way it deviates from all these recent futuristic shooters. Can't wait to see more and what inovative features are they bringing to us this time.

It's not so much that no one thought about exploring this era. It's more like nobody could figure out how to showcase the American military in that era, since they hesitated to join in (to maintain neutrality) until later in the war. Simply put, WWI wasn't as special in American history compared to say, the French, British, and Canadian. Since most shooters are supposed to involve the U.S. in some way (I have to admit I'm surprised the Call of Duty: Modern Warfare series toned down America's role and relegated them to supporting characters, but they were still a requirement to be involved in some significant way), WWI simply isn't a suitable setting for putting in American bad @$$es to be awesome and stuff.

I haven't even forgotten about the possibility. I still remember seven years ago, I was talking to a friend, and he complained about why we don't have any WWI shooters. The only ones we had were old-school titles that predate PS1 (I don't remember what they are, only he remembers). He specifically said, "I wish we could have a WW1 shooter just so we can have some f***ing variety for once. I'm bored of modern shooters and I'm bored of WW2. Does it always have to be about the U.S.?!" We were chatting about this while playing Modern Warfare 2, so you can imagine we had this setting in mind a loooooooong time ago, long before DICE even considered the idea. It's not an out of the blue idea, it's more like DICE figured out how to make it work with what the audience desires. And I imagine there will be a decent enough American influence to meet whatever unspoken market criteria there is.

WW2 had a lot going on, paratroopers landing behind enemy lines, spy networks, code breakers, railway cannons, blitzkrieg, huge technological advancements in weaponry, troop transport and fully realized tank warfare which could push across landscapes war had never touched before, and it was all documented by the soldiers themselves and journalists with a lot more cameras.
WW1 was a lot more grounded, slow moving, awful and hellish, DICE appear to be flourishing it up a hell of a lot but that's ok, i'm not that much of a stickler for historic accuracy that i'd be against this, i do want to know what that cape is about though.

So, as this post is about the trailer and there's a feud going on between DICE and Call of Duty, i think the COD trailer wins, more gameplay, better music, better story.