I love it so much I created this unfinished enthusiast resource:

http://united.imkeww.com

Material and Immaterial Reasons for Loving "The Mainline" or "The Friendly Skies":

The planes - most well rounded fleet

The widebodies - the biggest and best

The comfiest Business Class seats

The domestic C-class product on hub-hub and other major routes... great food!

A well-rounded route system - most major economic destinations represented

The new marketing and brand imagery - soothing shades of blue

The perception (though not necessarily true based on yield) of it being "the businessman's airline"