Alfresco guys has been broadcasting about restricting Site Create permission to users. Earlier all users could create Site on their own. There was no feature or permission set to control who can create Site and who not.
However, in 3.4, they have introduced a mechanism to restrict this.
But is this solution good? This mechanism was there since long, this is not new in 3.4. However, in 3.4 you can manage this using Share interface - no need to go to the Web Client. What you need to do is to revoke the Contributor privilege from EVERYONE from Sites parent folder, and when a normal user tries to create a site, a nasty error is presented. It would have been better if the Create Site link itself disappear in case a user does not have permission to do so.