‘salient’ in this case is being used in the military sense: if you have two armies facing off along a front-line, a salient is a piece of land that you hold that projects into the enemy’s territory (or vice-versa). An example would be the notorious Ypres Salient. Someone writing in 1936 would still remember the salients of the Western front, and would expect their readership to recognize the reference.
The thing about a salient is that while it’s advantageous in the sense that you can use it to push through the enemy’s lines, it’s also very vulnerable. If the enemy can attack at the base of the salient, they can pinch it off and encircle your troops, which pretty much never ends well. Salients tended to be the most active part of the battlefield in WW1, and the scene of some incredibly bloody battles.
I think what the writer is trying to say is that attempts to make social progress are over-extended in certain areas, and that – in their view – the job of an industrial psychologist is to help create a more even ‘front line’, i.e. unified social progress rather than piecemeal.
Elon is presumably using ‘salient’ as an adjective rather than a noun, where it means something between ‘relevant’ and ‘significant’; the salient pieces of your work would be the ones that actually matter.
