While there's overwhelming similarity between the operating systems in most cases, there's a lot of differences. As you probe more into the differences, you find that they emerge from deep-seated disagreements. Some are disagreements over development methodology, some over deployment and usage, some about what's important, some about who's important, and some about which flavor of ice cream is superior. Just comparing the surface differences doesn't tell you anything; it's the deeper differences that both explain and justify why each group does things the way they do.
Here are some of the reasons why (according to the FreeBSD.org website):
1. Supports a variety of platforms: x86 compatible, AMD64, Alpha, IA-64, PC-98 and UltraSPARC® architectures
2. Open Source - available free of charge and comes with full source code
3. Well-suited for a number of desktop and server applications
4. Can be installed from a variety of sources
5. Extends the 4.4BSD operating system feature set:
- merged virtual memory and filesystem buffer cache
- compatibility modules - to run programs for other OS including those for Linux, SCO UNIX, NetBSD, and BSD/OS
- Kernel Queues - programs respond more efficiently to a variety of asynchronous events
- Accept Filters - improve performance by allowing connection-intensive applications (ex. web servers) to push part of their functionality into the OS kernel
- Soft Updates - improved filesystem performance without sacrificing safety and reliability (analyzes meta-data filesystem operations so they're processed more efficiently)
- Support for IPsec and next-generation Internet Protocol, IPv6 - improve security in networks
- kernel support for stateful IP firewalling, as well as IP proxy gateways etc.
- supports encryption software, secure shells, Kerberos authentication, "virtual servers" created using jails, chroot-ing services to restrict application access to the filesystem, secure RPC facilities, and access lists for services that support TCP wrappers
BSD license is much more open than the Linux license :
Unlike Linux, with its many distributions, BSD has only 3 open source flavors that (vary according to focus) are all stable, secure and usable: OpenBSD (security), NetBSD (portability), and FreeBSD (usability and stability), which is ideal for x86 architecture
the kernel and drivers are more stable than Linux (although it takes longer to get the drivers installed, they are more reliable).
almost no loss of usable applications as FreeBSD package and ports system is consistent and easy to use with many open source applications available; offers the same crash protection of a journaling filesystem without much overhead (via its native filesystem, FFS, and the Soft Updates extension)