Good shared storage (able to be recommended) has, at a minimum, the following characteristics:

1. Safe (RAID 10)
2. Replication (open or proprietary)
3. Connectable (minimum of 1 GbE connection)

Note that performance isn't mentioned. Many usage scenarios don't require high performance. It's up to the end user to make sure the unit can handle the performance required by the application.

ReadyNAS Pro by Netgear

• RAID 10 available on more recent firmware
  http://www.netgear.com/images/ReadyNAS_Pro_Series_DS_26Apr1118-13987.pdf
• Linux rsync under the hood for free or ReadyNAS Replicate commercial solution
• Support for commercial backup products as well as cloud storage backup
• 2 10/100/1000 network ports with link aggregation and failover
• USB 2.0 connection
• Windows Active Directory support

Synology

• RAID 10 supported
• Several built-in backup/replication solutions, including cloud storage
• 1-4 GbE ports with aggregation
• Some models can add in 10 GbE cards
• models with USB 2.0, 3.0, and eSATA available
• SD Card on some models
• Windows Active Directory Support

QNAP

• RAID 10 supported
• Real-time replication to another appliance, rsync, and cloud storage backups supported
• 2+ GbE ports
• Models with 10 GbE available
• USB 2.0, 3.0, and eSATA available
• Windows Active Directory Support

Buffalo Terastation by BuffaloTech

• RAID 10 supported
• Has a replication feature to a second device
  http://www.buffalotech.com/technology/buffalo-advantage/replication/
• 2+ GbE ports
• USB 2.0
• Not all models support iSCSI

drobo

• drobo doesn't support RAID 10, only coming with parity RAID. Parity RAID is bad.
  http://spiceworks-faqs.john-refactored.com/doku.php?id=storage:parity-raid