Poopsy from Steam Forums summarizes the article in PC Gamer regarding Half Life: 2, Episode 2.  Apparently this information is official and exclusive, so you can be assured of the integrity of this post.

 - *NEW* Release estimate: Spring 2007
- Gravity gun will get new functionality
- Alyx is alive
- Vortigaunts will be more fleshed out
- More details about GMan will be revealed
- New vehicle will be introduced made by some guys in the woods. Will serve as enemy rammer =P
- You will cover the same amount of locales as in hl2 and the new locales will have a much more realistic feel to them since they are based off real locations (missile silo, mine shaft, etc.)
- Eli and Kleiner have set up base in the missile silo and you have to get the packet to them
- We’ll see the UBER and most difficult physics puzzle to date in episode 2
- New weapon known as Strider buster
- Strider and Hunter now has a weak spot *could change*
-Big battlefield-esque combat engagements with lots of enemies and allies fighting. Woods provide plenty of cover
- OH! And new yet-to-be-announced multiplayer game will ship with episode two…might it be TF2 set during the 7 Hour War?
- … (Any parts I forgot =P)