From my experience i would say its pretty balanced. I have been able to find grp for dungeon easily. Even though im low lvl (30ish) I have always run into ppl in the world from both faction and about same lvl.
No, its not balanced. Most part of the day, thorium point, menethil harbor, chill camp etc are camped by horde. Many alliance around as well, we fight back and you can have a great time being the underdog. You will have a hard time questing in high lvl areas but when lvl60 plenty of action. You will always find a group for dungeon or pvp.