Perhaps the ideal solution is one that omits these pages from search results UNLESS the user specifically requests them in some way (for example, a search term including "site:en.wikipedia.org")... but I kinda doubt that's realistic, on my part.
I'll be sorry to lose some ability to find information easily, but it's an unfortunate conundrum that articles and discussion pages are given the same weight, and absent some better way to fix the resulting problems... at least our internal search capabilities have gradually been improving, as I think has consistently been the main worry when this has come up in the past.
Should policy/guideline/essay/help pages continue to be indexed? It primarily seems to be process/activity pages which are problematic in this regard.
Are user pages uniformly problematic, or mainly those for banned/blocked users? If it's an isolated group at all, we could just as easily use the __NOINDEX__ keyword in relevant templates.
-Luna