What are some other areas or skills you think are paid too little attention by modern developers?
From my own experience, I wish I understood Linux better, as well as hardware. Not sure that would help me a lot at work though.
On a related note, maintaining a daily engineering journal. This was something I learned from older engineers when I started and it was through osmosis. It took awhile to buy into the value of it, but my memory isn't what it was and it's invaluable today.
And on a final note, estimation. Juniors should focus on accurate estimation of their work. Technical knowledge is pretty easy to find and acquire, but good communication and estimation are the two "soft" skills that grease the wheels of your career.