Speaking as someone who's generally strongly critical of JS abuse and overuse, Wikipedia with JS enabled offers a number of useful positives:

- Dynamic tables which can be sorted by column. (This is a feature I'd love to see baked in to HTML browsers generally.)

- The recent site redesign offering either visible floating or collapsed page navigation.

- Previews of both articles (Wikipedia links) and citations, such that these can be viewed without having to click through to a new page, or scroll the current article.

There are a number of others, I'm sure, but these three alone are quite handy.

Citation previews are nice, much more convenient than clicking the citation and then trying to find the back button. Link previews on desktop are useful too.