A few thoughts:

I've seen that Heinlein quote lots of times and I must say... I think it's a bit applause lights-y. Was there ever a time in human history when humans weren't de facto specializers? I believe that a small enough society could have a great deal of skill overlap, especially without educational institutions to obliterate intrinsic motivation and squash children's will to learn. That said, I think it's a good idea to apply effort and intention to the problem of living an interesting and flow-filled life. I'm just worried about people getting grade-hungry and losing their peripheral curiosity.

In general, I think it's better to treat the un-landscaped parcels of your mind and experience according to permaculture principles, instead of mowing it all to a uniform height and manicuring it like an industrial park. Then again, I might be rationalizing my own (hated) short attention span and inveterate laziness.

I'd like to see this thread turn into a series of postings about how individuals have leveled up according to their own specific goals. For example, last year I decided to improve my concentration. This helped. Of course, I also emptied my life of unwanted responsibility as of this past January, so that might be a more salient reason for my improved concentration. I suspect that time affluence might be the best overall hack for leveling up, but it's also one of the hardest ones to persuade people to try.