[The following analysis and trailer contain significant spoilers for Marvel's Agents of Shield Season 1. The first season, particularly the later episodes, where it crosses over with Captain America: The Winter Soldier, is worth watching ahead of Season 2.]
The teaser mostly contains footage from Season 1 (which is fair enough, since not much of Season 2 will have been shot yet, and they don't want to give away too much--besides, the point of a teaser trailer is mostly to convey tone, which this does). However, we do get a few new shots, such as Agent Skye being a ninja. Skye's come a long way from her Rising Tide days, and Season 1 hinted heavily that there are significant revelations and changes in store for her character.
Couson's (Clark Gregg) speech in voice-over is awesome, despite being made up entirely of cliches:
"This is a make-or-break moment. The enemy has the advantage. But we have to fight on,