A lot. As virtual server environments become more dependent on networked storage (NAS, iSCSI, SAN), the flexibility of being able to migrate VMs from host to host depends on storage access; although it would be nice, you can't have all servers access all storage. Typically, you'll want to stick with a single switch vendor and a single HBA vendor to decrease the chances that you will have incompatibility issues around drivers, microcode, and firmware. Once you have standards for storage, HBAs, and switches or directors, upgrades are less complex and predictable.
This was first published in December 2007