What can you export as a resource, using a NFS server?

Anything; that is any block device. CDROM, USB thumb drives, parts of your hard disk (or your whole hard disk if you are brave).

There are a few "problems" with NFS. Firstly, it is only a Unix/Linux based service. In other words, it is relatively difficult to provide NFS to Windows PC's. Technically this can be done, but the Windows client needs to run NFS client software. There are some options here, namely PCNFS and Cygwyn and these will allow Windows clients to "mount" the NFS exported drive locally.

Secondly, NFS is so dependent upon the network that in the event the network becomes unavailable, the NFS clients may wait for the service to be restored, and this may cause the client to "hang". While this is not critical (it will not mean the client needs to be rebooted) it will cause the machine to slow as it tries to "find" the missing file system. For example, doing a df -h while the NFS server is unavailable causes that terminal session to "hang". The rest of the machine is still available, as long as you do not make use of the unavailable file system.