DRBD gives me a block device that's mirrored on 2 or 3 machines. DFS is like a file-level replication system or something. For instance, I can use DRBD as an iSCSI target for instance and then failover SQL Server. Or I can use any other app that requires no possibility of conflicts or uses open files.

AFAIK, Microsoft still has no solution in this case but to introduce a single point of failure via a SAN or other shared storage. I'd love to be wrong.