In the end the interface you like is the one your are used to. The problem is that during the early part of your life you learn things at an exponential rate and everything is easy to understand and work with. later in life its harder to learn things no matter how you try.
Example, if your born in the USA and move to france to live when your 8 yrs old you will end up having the same french accent as somebody born there. Try the same thing when your 40 and even if you learn french it will be with an english accent easy to spot.
I grew up with dos back when the xt was invented, using dos is easy for me and I understand it and how batch files work, prompts, etc. Somebody who grew up using the apple mac has no clue how a command prompt works so they might not feel comfortable using it in OSX on their new machines.
So even if somebody invented the easiest to use operating system some peaple will have problems with it because of their past computing experiances. I also think that is the same reason Amiga and mac fans hate the pc.. they just didnt grow up using one and dont use it enough to learn it (thats the reason I dont bother with linux)