Valve has stated their opinion regarding the recent changes in YouTube video policy. The developers are strongly encouraging fans to use content from their games in a variety of videos. Players will be able to upload, stream, comment and edit video footage from Team Fortress, Portal, Dota and more, as long as they use the content separately or do not license their videos for additional payments.
The company behind Half Life does not like the new YouTube Video Policy and says that as long as users do not charge other people for the viewing of their videos they should be fine. Fans will be able to monetize their videos, as long as it does not include additional charges to the user.
They are also adding that people should not be contacting them to write to YouTube, but rather send links of the message to the YouTube copyright department. That being said, as long as you are not using any additional content, which you do not have the rights to (e.g. music, movies) you will be able to stream content from the games of Valve and monetize it.
Read The Official Statement From Valve