Mindfulness meditation seems like another good example, especially since the required time investment for you to start seeing benefits seems to be pretty large.
My experience with mindfulness meditation differs from the standard narrative. Once I had practiced long enough to be able to meditate for 15 min. with no problems, I found meditation much more useful for inducing a concentrated state or taming bouts of internal turbulence than any longer-term effects.
So, transferable skills: skills that, upon improvement, increase your ability in other areas (and also improve other, higher-level skills).
A basic example would be reading/writing. Knowing how to read and write allows one to access a huge amount of other skills and resources which are otherwise unavailable. A less obvious example would be clear speech (enunciation). Ability to speak clearly improves one's prospects in a lot of different areas (e.g. professional advancement, dating, etc.).
I'm looking for additional examples. Which skills did you find to be most transferable? Did you become proficient in X, and then found this helped you in many other areas of your life? Please share.
(I tried to find whether this was discussed before, and failed; if it was, I would appreciate the link.)