Trailer was awesome, I especially loved the prt where the dreadnought was like "I have come to destroy you" then torches the banshees head.
As for the space marines getting massacred, it makes sense, in all fairness the second wave should have been slaughtered as well. I know space marines are championed in the fluff because they are the "good" guys in the universe of 40k, but the eldar they were fighting here were a lot of tougher guys. They had a couple exarchs and a farseer, if it was the tabletop game the only thing the Eldar would have had problems killing would be the dreadnought. If anything, its a big stretch saying a veteran sergeant could kill a farseer, she would tear him apart normally lol.
I agree with the people saying its going to be Tyranid, I don't think it could be anything else really.
Also, Eldar are basically future elves, so yeah tha's why the farseer has a human face and pointy ears.