The "special features" are temple ordinances for the dead. Only members of the church can perform these ordinances, and so if you are a member of the church you link your family search and church accounts together so you can reserve ordinances. That's it.
If that doesn't make sense, let me spell it out in a lot of detail.
We believe that we can perform vicarious ordinances (catholics call them sacraments) such as baptisms for the dead. People who are living can go to the temple and receive baptism on behalf of their ancestors. In order to do this, they must be a member in good standing. Then they print out record sheets indicating who the ordinance is performed for. They take those records to the temple, perform the ordinance, and then the temple records that the ordinance was done. Then the records in the family search database get updated indicating that the work was already done so it doesn't have to be done again.
We believe that at the last day, Christ will ask us to deliver the records of all the ordinance work done since the beginning of time. We will give him a book containing the names of all the people who have had their ordinances done. When those people are judged before God at the final judgment, they will look up their name in the book to ensure that their ordinances were performed. If so, then they are eligible to enter the highest degrees of glory and exaltation, contingent on their faith in Christ alone.
That is the only feature that isn't available to people who aren't members of the church, and the only reason is because they can't do those ordinances for the dead until they become members of the church.
If that doesn't make sense, let me spell it out in a lot of detail.
We believe that we can perform vicarious ordinances (catholics call them sacraments) such as baptisms for the dead. People who are living can go to the temple and receive baptism on behalf of their ancestors. In order to do this, they must be a member in good standing. Then they print out record sheets indicating who the ordinance is performed for. They take those records to the temple, perform the ordinance, and then the temple records that the ordinance was done. Then the records in the family search database get updated indicating that the work was already done so it doesn't have to be done again.
We believe that at the last day, Christ will ask us to deliver the records of all the ordinance work done since the beginning of time. We will give him a book containing the names of all the people who have had their ordinances done. When those people are judged before God at the final judgment, they will look up their name in the book to ensure that their ordinances were performed. If so, then they are eligible to enter the highest degrees of glory and exaltation, contingent on their faith in Christ alone.
That is the only feature that isn't available to people who aren't members of the church, and the only reason is because they can't do those ordinances for the dead until they become members of the church.