IMO and old-man experience, trolling went off the rails after a few years when sites like Something Awful got really huge, and naturally the lowest common denominator wanted in on something 'cool' that they mangled badly. Trolling morphed into"let me spew sewage at a random website / forum / blog to insult as many people as possible, directly, and then run back to my friends and claim I won the internet."
Like anything, 'trolling' can be used as performance art, but personally I tend to think it only really has legitimacy like that when it's for a noble goal, ironically. For example, while the real-life behavior of a lot of 4chan's scientology trolls turned stupid real fast, stuff like the Anonymous "we run this" videos taunting Scientology were artful and genuinely amusing.
In my mind, that is this 'artform' trolling.
Most of the time, aside from just throwing insults at people to get a reaction, people today troll by pure disruption - i.e. that guy who comes into a forum thread to take a shit on what he sees as the popular opinion in a passive aggressive manner, intended to insult everyone there.
Increasingly though, aside from a few very high minded and carefully constructed pranks / campaigns, it appears that enough people are on the Internet today that trolling in general just comes off as childish. A lot of trolling, even by its original definition, was predicated on the notion that the Internet was just bunch of stupid, one big video game that only dumb people invested anything in. That was when the Internet was still a toy, and people could rationalize that most of real life (and thus 'serious business') had no intersection with the net. You still see a lot of people with that attitude today, but I think they're behind the curve. The net is maturing fast, has become an extension of real life, and thus taking a random shit on it is becoming like walking up to someone in real life and taking a random shit on the sidewalk in front of them. It's ridiculous.
One might argue that places like the chans are greasetraps for people who still want to use the internet as a solipsistic playground for a sort of simulated sociopathy.