Problem: the hopelessly inefficient use of space in the Watchlist (below is relative to enwp, but the issue is general to Mediawiki across projects).
On a brand spanking new high-end laptop with a high-res 15" screen, the usable space of the Watchlist is as follows:
That's a hell of a lot of wasted space that could have been used for the content I actually care about with a few simple changes (bonus points for hiding/minimizing the left sidebar, but politics will shoot that down even as a user preference off by default):
The effect on usable space would be pretty dramatic (especially if you could move the article title, "Watchlist", up above the tab bar, "Special", or where "Read"/"Edit"/etc. are for mainspace articles):