Jira currently has the ability to link issues together.
The links have a qualifier describing the type of link e.g. Blocker, Reference, ...

When viewing an issue you can see the "links" of all issues that are 1-node (or link) away.

What is missing is the hole picture.

A "Link Map" or something similar would be a great add-on and will give all project leads a better understanding of the "dependencies" in a given project.

Now since jira already has a great useability rating with all the well thought off icons/colors/etc. I suggest that the "link type" gets a new attribute "color" or even an "icon".

If you then construct a graphical view of the dependendy map, you can colorize the "lines" (links) between the Issue-Symbols. Adding even more information about the dependencies in your project.

I believe that once you start working on the graphical representation of the dependency map more ideas will come up.

As a sidenote... having a feature like that, will put jira in competition with every requirements engineering tool. Why? Because I have only seen dependency-maps in tools like Rational-Rose, CaliberRM, etc.