RAM

The MBP comes with a meager 4GB of memory.  I can analyze all the memory usage all I want and decide if I need 4GB, 8GB or actually 16GB under OS X Yosemite.  But I guess I have something better to do (actually I don't) than analyzing memory usage and reading the Activity Monitor.
PS:  If you have a retina display MBP, congratulations, you are stuck with whatever memory that comes with the machine.  No upgrade can be done to the RAM, short of buying a new one.  More power to Apple.  The mid 2012 non retina MBP models are quite possibly the last machines that are user upgradable.