I believe that the most LWers have some STEM background, so they are already familiar with such level of criticism, therefore criticism-is-normal disclaimers aren't necessary. Am I wrong? :)

Possibly, given that lukeprog, Eliezer, and Yvain have all complained that writing LW posts is not very rewarding. Reframing criticism might do a bit to mitigate this effect on the margin :)

Positive reinforcement is a thing. But how are you going to efficiently encourage readers to do that? :) Also, we have karma system, which (partially?) solves the feedback problem.

One of the things that strikes me as interesting reading Eliezer's old sequence posts is the positive comments that were heaped on him in the absence of a karma system. I imagine these were important in motivating him to write one post a day for several years straight. Nowadays we consider such comments low-signal and tell people to upvote instead. But getting upvotes is not as rewarding as getting appreciative comments in my view. I imagine that 10 verbal compliments would do much more for me than 10 upvotes. In terms of encouraging readers... like I said, put it in the FAQ and announce it in a discussion post. Every time someone sends me an encouraging PM, I get reminded to send others encouraging PMs when I like their work.