Legs had gone, awful finisher, can't head a ball , couldn't catch a cold are all fine all football based criticism of a footballer. But don't kid ourselves that it stops anywhere near there just look at the abuse Ben Turner after a couple of high profile mistakes and shockingly bad games! It went way past that.
If contained to 90 mins once or twice a week like back in the pre internet days I'm sure there would be very few complaints. Abuse to players , managers, officials has been commonplace and part of football and the terrace culture. However nowadays it's 24/7 , it's on multiple media, there is of course no right to reply (as we know that doesn't end well when the player or manager answers back, it's a no win for them). It must be hard to avoid unless you adopt zero exposure.
It's easier to go too far behind a computer screen or on your mobile than it is when sat at Meadow Lane or stood on an away terrace surrounded by people.