I think it would be valuable if a sort of karma-like level would be associated to all users. It would be calculated by multiple factors, such as how long the user is established, some sort of ratio between contributions and warnings and others such as length of edits. This way, in the "Recent changes", a karma value for each edit would show up, a bit like the size difference currently shown on the page..
Say a user is there for 4 years, has 10000+ edits and only a few warnings, when he edits a page, in the "Recent changes" log, it would appear as a high karma level, in an descriptive colour (blue). If a newly registered user who has only 20 contribs and 10 warnings edits a page and removes two paragraphs, the number would be negative and in a "warning colour", such as red.
Disregard the name, (karma) I found nothing else more appropriate to name it as.
Maybe it would be more appropriate as an extension, but I feel it's valuable enough to be integrated in Mediawiki.
I think it would help greatly in fighting spam and vandalism, as contributions from established users would then be easily discardable when looking at the "Recent changes" page. It would also be easier to spot malevolent edits.