It would be really useful if there was a feature to "bookmark" Wikipedia articles provided by the site itself.
I've got hundreds of Wikipedia articles I'd like to read (too bad there's no "Nuremberg Funnel" yet ;-) or like to have at my disposal (for discussions, coming posts of mine, etc etc).
So I guess people in here would immediately say that there's already the bookmark-function of Browsers so it's a redundant request. However: bookmarks of a browser are machine & browser-dependent. Especially with more and more people using Wikipedia on their mobile-phones and multiple devices at once that's not an argument to underestimate. Also many other sites have this function and it's getting much used there - it's not implemented and used there without reason.
Furthermore Wikipedia could allow for many features and augmentations to this bookmark-list (for example for organization/sorting of them but there are tons of possibilities! [not just by Wikipedia itself but also by extensions/tools - from notification-systems to categorization-systems to recommendation-systems etc]). Additionally this would be an incentive and functionality that would invite more people to (contribute to and get familiar with etc) Wikipedia.