The human behind it, the community using it critically, and the years of battle hardening.

The great open source tools out there have handled, worked around, or influenced away many many bugs and edge cases out in the real world, many of which you wont think of when initially designing your own. The silent increase in stability and productivity resulting from this kind of thing is as vast as it is hard to see/measure. It feels like the quote about expertise saying someone "has forgotten more than I'll ever know about [subject]".

Thank you to everyone powering our collective work.