The release of the first Dire Straits album is a turning point because it made Mark a genuine recording artist, songwriter and guitar player, and it meant he could focus on music as a full time career.
Bob Dylan asking Mark to play on Slow Train Coming is also a turning point, being asked personally by his songwriting hero so early in his career must have blown Mark away, it lifted his profile hugely.
During the recording for Making Movies, David Knopfler left the band, this must have had been a turning point for Mark, he no longer had the sibling rivalry to contend with, and it meant he was the only person called Knopfler in the band!!!
Recording his first film score for Local Hero is a turning point as it gave him a different outlook and focus, he could do something away from the group as a sideline, and he found he is very good at it. Film score composing is a different approach from what he had been doing up to the point in his career.
Getting a call from Chet Atkins, Mark had already met and worked with his songwriting hero, now it was the turn of his guitar playing hero, and again it was his hero that approached him, Mark was already on his way to being a guitar playing legend, but to then have someone of the stature of Chet Atkins praise you highly and want to work with you must have been a huge compliment for Mark, it also must have made Mark raise his playing abilities further, we know Mark is self taught, but if he ever came close to having a tutor, then it was Chet.
The release of Brothers In Arms, it made Dire Straits one of the biggest musical acts of all time, if that is not a turning point, I don't know what is.
The On Every Street tour signaling the end of Dire Straits, by this time Mark had enough of the size of the band and the size of the tours, he decided to be a full time solo artist, so this was a turning point both personally and professionally.
The release of Golden Heart was another turning point for Mark, as this proved he didn't need the Dire Straits name, he could still make it under his own name.
The death of Chet Atkins in 2001 had a huge impact on Mark, and I believe a turning point in his guitar playing, as he didn't need to prove anything anymore, he didn't need to be good enough to keep up with Chet anymore, and meant he could now focus on being the song writer he always believed he wants to be.