This trailer shows some evil menacing force scanning Master Chief and revealing the process in which he was created, and also shows a glimpse of the new threat (Prometheans), while giving a glimpse of the larger threat, this new imposing foe who comes face to face with the Master Chief.
That makes the average person curious. Makes them ask who that guy scanning the Chief is. Shows them a glimpse of the Prometheans. Shows them some of the Chief's past and his creation.
Those are all buildups. You can say it wasn't as thrilling, and didn't have a great action scene. BUT, it does make the viewer ask questions, and get excited about the bits and pieces they're receiving about the story in Halo 4. The Chief finally comes face to face with a force as strong as he is. That works. That builds excitement.
I find that everyone I showed this too loves it. But people who already know about these things aren't as excited because it doesn't make them any questions. So they see this as lame. But in your case, its just supposed to be some eye-candy and fanservice. But you should know that regular people don't know about the Chief's past, or that there's a specific new antagonist.