"The Problem" is how to enable users who have a low reputation to post good/valuable/worthy comments.
{I realize that this is not a new topic, but I believe it is a new proposal, and the previous proposals have all been marked as "status-declined" for some time. See here for the most complete earlier discussion I found. I am posting this separately so it can be hated and down-voted for how it solves The Problem, rather than for how it solves the problems with previous suggestions.}
It's worth emphasizing that my primary goal is simply to stop the current practise of turning away good content "just" because the source of that content has a low rep. I don't have general data, of course, but I can say that I personally was driven to solve this problem when the ratio of good-answers/comments-I-was-not-allowed-to-post to good-answers-comments-I-was-allowed-to-post hit 10-to-1.
My suggestion has many variants; but here's the simplest and best:
"Sponsorship"
Allow low rep users to post comments and answers, but make these visible ONLY to users whose rep is high enough that THEY could have posted that same comment or answer. And then allow any one of those high-rep users to "sponsor" that post with a single click: a sponsored question or comment is visible to the entire community, marked as coming from "foo, sponsored by bar". Up-votes and down-votes will be applied to BOTH accounts, which is the obvious way to reward good sponsorship and deter any kind of nepotism or special-treatment on the part of a potential sponsor.
There is no need for any relationsip of any kind between low-rep user and sponsor. And there is no suggestion that the sponsor was in any way involved or responsible for the sponsored content, other than having clicked once to tie his/her reputation to that of the sponsored content. (Of course, a high-rep user can always communcate with the author of any content s/he sees. That includes low-rep authors of potentially sponsorable content. Any such communication is completely outside the scope of this proposal. Undoubtedly there will be high-rep users who torture low-rep newbies with endless edits and tweaks to answers, dangling in front of them the carrot of sponsorship. But generally, the answers will continue to be the sort of single-author/one-point-of-view that I think is a real strength of these sites).
There are many many variants of the idea, but as far as I can see, all of them are more complex than this to understand and implement. Perhaps even more important, all of them would make a larger change in the quality of the content that one sees here. Today, one sees "content offered by an invidual who passes the test of having given a certain amount of appropriate content in the past, and who is willing to stake his/her reputation on this new content as well." My proposal doesn't substantially change that, and that's important.
I won't say that it's utterly trivial to implement, but it's pretty damn simple -- certainly much simpler and cleaner to add than any system that involves voting or some new approval structure. Questions and Comments of newbies or other low-rep individuals don't have to be "approved" by anyone; they just have to be "sponsored" -- someone who actually could ask a question or make a comment needs to indicate that "gosh, I wish I had said that". (Well, at the very least, that "gosh, I could have said that.")
I note that no moderator will be harmed as a result of this scheme. Their lives are utterly unaffected (other than the fact that they are likely to find themselves moderating a great deal more content if it is deployed).
As a final comment, I'll also note that to remove all possible objections to the proposal, perhaps one might implement an "opt-in" or "opt-out" check-box. For low-rep users, not opting-in (or opting-out) of the scheme would mean that they'll just have to continute to answer unprotected questions in order to get full privileges. More importantly, for high-rep users, opting-out (or not opting-in) would mean that they will not see the posts of low-rep people at all, so their experience would be unchanged compared to today's. (Except, of course, that they'll get the benefit of the fact that good answerers and commenters will increase their rep much more quickly as a result, which will increase the good content available to all, even the opt-outers). I certainly don't recommend doing opt-out/opt-in for low-rep users. Even for high-rep ones, I think it should only be done if there are LOTS of complaints from established, high-rep users that they just don't want to see any chaff-with-wheat in their Stack Exchange. I don't for a moment deny the existence of chaff, and the need for tiers of participation based on reputation. But I'll bet that there is a LOT of content that everone is missing because the current quality-control mechanism is an exceedingly blunt instrument.