1. Disk encryption is relatively useless on anything outside of laptops. It also offers little or no protection against remote attacks
2. Apple still has not addressed the fundamental flaws with the package management system, paths to root, weak sandboxing, and improper/partial implementation of DEP and ASLR.
3. Auditing is still suspect on any system that does not support CAF functionality, ad that to the lack of a reference monitor and the logs are not truly trustworthy.
4. The management utilities have had mixed reviews. SOme poeple love it, others absolutely despise it due to reduced functionality.

Most of the admins I know are refusing to upgrade and advising clients to switch to more secure/manageable Linux or Windows platforms.