Each person that 'signs up' has access to a limited number of forum threads initially. This is a spam prevention measure. In order to get full access to all threads it is necessary that people post at least one entry in the introductions thread giving a little bit of information as to where their interest in LEGO is etc.
Another reason why you may not be given full access is if your profile information is incomplete, for example no 'real' name or location (it doesn't have to be exact or an address) has been provided to the site admins.