I don't really have time.
Security: The guys who competed at Pwn2Own said OSX security is a joke and they haven't fixed anything in Snow Leopard. OSX is (and always has been) the first to fall, and Safari is the most insecure browser on the market.
There have even been security bugs in HFS+! If you run a web server you have to add specific hidden files and hidden folders to your server's deny list, otherwise a user can read (or possibly change) the source code of your web apps. As far as I know this is an outstanding bug.
General performance: In thread scheduling performance Linux is king, followed by Windows, followed by a barren wasteland, followed by OSX.
The main reason Windows shows a lower benchmark performance is because its thread scheduler is designed to make user interface threads more responsive. Linux and OSX don't do this. OSX really is the biggest loser, whether you're running a GUI or a dedicated server.
Game performance: To put it simply, hardware calls are expensive. OpenGL is designed to split the display driver into two parts: one part that runs in the kernel so it can access hardware quickly, and another part that runs in the application so it can optimize hardware access.
On Windows and Linux, the GPU vendor supplies both parts. That way performance is optimized for the specific hardware you're using.
On OSX, Apple provides the application half of the driver. GPU vendors are forced to implement one inefficient kernel interface.
Between this and the threading issues, WoW performance in OSX vs. Windows has historically been as low as 50%. Blizzard has optimized some things (sometimes even disabling things,) and Apple has done some work on performance, but there's still a huge difference.
Could keep going, but you get the idea.
(Edit: To give some anecdotal evidence of this, back in 2008 I ran the demo of the first Penny Arcade game on my brand new 8800 GT, 2.8 GHz octocore Mac Pro running Leopard. The framerate was so slow and choppy the game was unplayable - and these were definitely not detailed graphics for the time.
At the same time, my girlfriend was playing it on her 4 year old Athlon X2 system and something like a Radeon X1600, running XP. Totally smooth.
It's not like this is some edge case app that was badly optimized for OSX, either. The game was built on Torque. I encountered very similar performance characteristics while porting my apps over.)