After he debuted way back in The Avengers' post-credits stinger, much has been made of the immense threat of Thanos without us ever getting to really see what he's bringing to the table. Sure, he's fearsome by reputation (and by the simple fact that he's able to recruit other supervillains to work for him) and we know he's large, but beyond that, he's just another threat to the universe.
It might have been easy to keep him mostly hidden from the trailers, treating him more as a big reveal (like the point where he walks through the black hole for instance), but instead, this trailer ingeniously puts him front and centre. He also comes with his own demotivational speech, designed to crush the inspiring spirit of the Avengers' call to arms at the beginning, and there's something just delightful about his snarling, apathetic delivery.
This guy means business, and that's the message this trailer perfectly sells.