Loki doesn't play the main villain here. That role is set up for another character from the Thor comics, and we got a very brief glimpse of him in the trailer. This is clearly just a teaser trailer.
+1 I'm glad they have Loki in, but not the main baddie. I loved the older Thor comics where they bothered to at least still show cameos of key characters. We should get an update on Loki, but don't want to see a lot of him again in this next film.
Trust in Marvel - they haven't let us down yet!