Integrate functionality from the best or most popular plugin packages. Put that code right in the core product, so every WP version "ships" with the extended features that users are demanding.
Examples: Ultimate Tag Warrior, Event Calender 3, etc. Complicated plugins that seldom feel as if they are very well integrated with WP out of the box...
One thing that has long irked me about WordPress is this whole philosophy of "if we can't do it there's probably a plugin for it"... now, on the surface, this is very true--many things one might want to do above and beyond the basic setup can be accomplished with a plugin. The problem is simply that many plugin authors don't maintain their code, and newer versions of WP aren't necessarily going to support the code of an old plugin. Not to mention the issues with quality control of code and the sheer number of plugins that need to be sorted through. Plugins are great--don't get me wrong--but it can become problematic to have to maintain a herd of twenty or more just to get some basic things done that could easily be accomplished by expanding the core admin options or some such thing.