By implementing something like http://wordpress.org/extend/plugins/rb-internal-links/ but in a more intuitive and prettier way, this would put WordPress in a great place as a leading CMS and blogging platform.
There are a number of times where one would have to link to another page let alone blog post when creating internal content. The intuitive and built-in internal linking mechanism would vastly improve a "regular" (ie non-techy/blogger) users experience.
For a CMS like WordPress, this is a surprisingly overlooked feature that is not in the core. Hope that helps make a great piece of software even better.
Thank you for all the work and effort so far!