I used to be like that ... I used to point others mistake till I found that I am not better ... I think as we mature we learn to embrace the fact that we are vulnerable too. I started to encourage those that I used to criticize. I started to learn to be better.

+1, I've made the same experience and completely agree. We should more often focus on the good parts instead of the imperfect ones.

I've also learned, that the more I'm bothered with a website/webapp and how it was created, I understand the reasons why this or that was done in a certain way (legacy data, which needed to be shown in a certain way, late design changes, etc.)

Well you know what Bill Gates said about lazy people didn't you ;)

I think about it quite a bit. What do you say to auditors, testers? from #a11y to performance - they are there to point out errors, improve a condition - by definition.
But I hear you. Tone is the key... gawd knows I make my own mistakes, and hell - Zeldman personally reminded me one day in a podcast. Made me think...