Just the default BP installation - pasting the Youtube URL will auto-embed the URL
I have done some troubleshooting and the only solution is to disable the Rate Forum Posts plugin
Here's a live post on one of my sites which shows the Youtube URL not embedding with the Rate Forum Post plugin activated
http://mymobilefest.com/groups/android-army/forum/topic/the-s4/
The video shows up in the activity stream but not the forum post.