Edit: This video is from early may 2013. Since then, many new discoveries have been made and both engines are more broken in more different ways, not portrayed in this video. The differences mentioned in this video still stand, but nowadays they are outshadowed by the findings since.
Edit: Damn, I forgot to put the airboat increased pick-up radius.
Basically you can pick up items from further away when in an airboat on new-engine, while in old-engine you need to be closer.
Lightning and fancy Shotgun soldiers aren't the only things they changed.
Here are some Speedrun related differences.
Might've forgotten something but w/e.
If you're looking for Old Engine, please hit me up with a twitch message (lotsofs). Edit: Dont do this anymore, simply google "source unpack" and youll find it
Music is from the Half-Life 2 soundtrack