If we throw in network devices and appliances, the patching percentage is even worse. I've yet to penetration-test a facility that had fully patched routers and security devices. The very devices they depend on for protection are the ones that appear to be the least patched. This fact continues to befuddle me, but it's true in most environments. I assume that stand-alone appliances are unwittingly considered "hardened" by most people, so they don't put as much emphasis on patching them. But even more, device and appliance vendors are horribly neglectful in creating mechanisms to automate the process of pushing down critical OS and application patches to their own devices. They patch and update the application running on the device, but not the underlying OS.
Even when you have the best patching process in place and patch everything, there will be some percentage of patches that fail to install. Do you test for that? Is your missed patch reporting accurate?
Don't log on as root or admin all the time
It's easier to say this than to accomplish, but no one should be logged in with an elevated account all the time. Every OS has a method of providing alternate, less-privileged logins, each with its own advantages and disadvantages. However you accomplish this, every moment you actively compute at reduced level while not accomplishing administrative tasks is an improved moment in safety.
Use strong passwords
No user password should be shorter than 8 characters. It's even better if they are 9 or 10 characters long. Elevated accounts should have even lengthier passwords. Passwords should not be shared between internal and external sites, and they should be changed every 90 or so days. Account lockout should be turned on. I don't care what the account lockout settings are, but the mechanism should be enabled to prevent easy brute-force guessing.
Of course, there are many more security tasks to accomplish than these four, but if administrators were better at this quartet -- consistently better -- the risk of malicious exploit would drop significantly.