I'm in your boat as far as mostly playing friends and computer. Beat the living hell out of both. (Won the Prem with Southampton!)
Online people play like jackasses. I played a guy who ran a custom formation with like two defenders, 3 midfielders, and 5 forwards or something like that. By the time I could even figure out what was going on, I was down 4-0, ended up losing 5-4, you deal with a lot of dbags who just bomb it down the field.
Also, people quit all the time. I am always good about taking my beating, but if you go up 2-0 a lot of people will quit.