I have merged the Gerrit change (https://gerrit.wikimedia.org/r/#/c/124475/), restoring the body font to "sans-serif". The heading font is unchanged for now.
I've read the entire wikitech-l thread (89 emails as of writing) and pondered this carefully. My summary of the situation is:
* This font stack, according to WMF Design, only provides real improvements for Macs (~6% of Wikimedia sites visitors per http://stats.wikimedia.org/wikimedia/squids/SquidReportOperatingSystems.htm) and should be (nearly) identical to defaults for other systems. However, it causes major rendering issues for an unspecified number of Windows and Linux users (especially with, respectively, Helvetica and Nimbus Sans L; see both open and closed dependencies of bug 63549). * This font stack also apparently causes issues with non-Latin-script languages (not very well-specified ones, though; more bug reports like bug 63817 would be welcome); it's serious enough for at least one affected Wikimedia wiki (the Japanese Wikipedia) to have already reset the stack to "sans-serif". This might affect the serif heading fonts more than the sans-serif body fonts (again, more precise reports needed). * The wikitech-l discussion, as well as various on-wiki discussions (e.g. on WP:VPT on the English Wikipedia) have been overwhelmingly in favor of restoring the plain "sans-serif" font definition. * Orthogonally to these issues, all other aspects of the typography refresh have been generally considered successful and minor problems with them have been quickly fixed.
Based on the points above and my own common sense I think this should be merged, and a similar follow-up for serif heading fonts might also be necessary (but that's obviously a lower-severity problem, there isn't that much text in headings). Steven, I'm sorry, but I'm overriding your -2.
(I'm posting this on the Gerrit change https://gerrit.wikimedia.org/r/#/c/124475/, on the tracking bug https://bugzilla.wikimedia.org/show_bug.cgi?id=63549 and in the wikitech-l thread. Please reply on wikitech-l.)