FC is available in 1Gbps, 2Gbps, 4Gbps, 8Gbps, 10Gbps, and 20Gbps speeds. Switches and devices that support 1Gbps, 2Gbps, 4Gbps, and 8Gbps speeds are generally backward compatible with their slower brethren, while the 10Gbps and 20Gbps devices are not, due to the fact that they use a different frame encoding mechanism (these two are generally used for interswitch links).
In addition, FCP is also optimized to handle storage traffic. Unlike protocols that run on top of TCP/IP, FCP is a significantly thinner, single-purpose protocol that generally results in a lower switching latency. It also includes a built-in flow control mechanism that ensures data isn't sent to a device (either storage or server) that isn't ready to accept it. In my experience, you can't achieve the same low interconnect latency with any other storage protocol in existence today.
Yet FC and FCP have drawbacks -- and not just high cost. One is that supporting storage interconnectivity over long distances can be expensive. If you want to configure replication to a secondary array at a remote site, either you're lucky enough to afford dark fiber (if it's available) or you'll need to purchase expensive FCIP distance gateways.
In addition, managing a FC infrastructure requires a specialized skill set, which may make administrator experience an issue. For example, FC zoning makes heavy use of long hexadecimal World Wide Node and Port names (similar to MAC addresses in Ethernet), which can be a pain to manage if frequent changes are made to the fabric.
The nitty-gritty on iSCSI
iSCSI is a storage networking protocol built on top of the TCP/IP networking protocol. Ratified as a standard in 2004, iSCSI's greatest claim to fame is that it runs over the same network equipment that run the rest of the enterprise network. It does not specifically require any extra hardware, which makes it comparatively inexpensive to implement.
From a performance perspective, iSCSI lags behind FC/FCP. But when iSCSI is implemented properly, the difference boils down to a few milliseconds of additional latency due to the overhead required to encapsulate SCSI commands within the general-purpose TCP/IP networking protocol. This can make a huge difference for extremely high transactional I/O loads and is the source of most claims that iSCSI is unfit for use in the enterprise. Such workloads are rare outside of the Fortune 500, however, so in most cases the performance delta is much narrower.