Different Lowpans will have different SecurityObjectives.
The obvious ones are:
integrity. Most Lowpans will need some form of integrity protection for the data transmitted. Note that integrity protection requires some form of authentication (typically, device authentication).
confidentiality. Not all Lowpans need this, as there is often little interest in protecting e.g. weather data.
Another important security objective is:
protecting the network. In particular, one DoS attack on battery operated devices is to deplete theur batteries by sending them large amounts of bad traffic. (This objective is an instance of the general objective of availability.)
Other objectives might include:
authorization. With certain lowpans, it is important to identify why different devices wants to communicate. In some cases it is dictated by the user during installation (basically the user intent to connect is the authorization) and/or the relationship of applications.
(Note that experience with WLANs has shown that relying on limitations of the radio signal propagation is rarely a secure strategy. See also Security.)