Below are some of the disadvantages of Mac OS:
1) The first one is offcourse the software compatibility as almost all the applications of Windows or say Microsoft are not supported by Mac OS. Due to this reason, you won't be able to use many of the 3rd party applications even if you have and wish to use them :(
2) If you are a newbie, make sure you will find it really very arduous to theme the operating system for your requirements.
3) It is considered to be the last choice if you are a gamer as most of the games available for Windows (which might also be supported on Linux) are not supported on Mac OS and hence you won't be able to get that gaming experience with Mac OS
4) Though, you will be able to modify the operating system with some of the 3rd party applications available, Mac OS is very limited when it comes to performing tasks if you compare it with Linux or Windows.
Hope this helps....