One that I've dealt with in the past was application availability. Sometimes the order varies, but products might be released on Solaris/NT first, then possibly AIX and HP-UX and sometimes Linux.
Supported tools. Solstice Disk Suite or Veritas Volume Manager for logical volume management on Solaris. The LVM in AIX is supported under the standard OS support contract. Is ReiserFS or EXT3 officially supported by the OS vendor as part of the OS since it can be considered an optional component? Linux has a lot of tools available, but how many will be supported by the vendor the OS was purchased from? Will you have to go to the person who wrote the open-source application/tool? Just a few to think about...
