Some of the new features are:
Integration of the ProPolice stack protection technology, by Hiroaki Etoh, into the system compiler. This protection is enabled by default. With this change, function prologues are modified to rearrange the stack: a random canary is placed before the return address, and buffer variables are moved closer to the canary so that regular variables are below, and harder to smash. The function epilogue then checks if the canary is still intact. If it is not, the process is terminated. This change makes it very hard for an attacker to modify the return address used when returning from a function.
W^X (pronounced: "W xor X") on architectures capable of pure execute-bit support in the MMU (sparc, sparc64, alpha, hppa). This is a fine-grained memory permissions layout, ensuring that memory which can be written to by application programs can not be executable at the same time and vice versa. This raises the bar on potential buffer overflows and other attacks: as a result, an attacker is unable to write code anywhere in memory where it can be executed. (NOTE: i386 and powerpc do not support W^X in 3.3; however, 3.3-current already supports it on i386, and both these processors are expected to support this change in 3.4).
Still more reduction in setuid and setgid binaries, and more chroot use throughout the system. While some programs are still setuid or setgid, almost all of them grab a resource and then quickly revoke privilege.
The X window server and xconsole now use privilege separation, for better security. Also, xterm has been modified to do privilege revocation. xdm runs as a special user and group, to further constrain what might go wrong.
To see all the changes take a look at the complete changelog.