I like being able to link easily from one issue to another (e.g. TST-19036) – we evaluated FogBugz and they have a great feature which is that when you mouse over a link to another issue, after about 500 msec you get a small popup showing a summary of the linked issue. JIRA does show the summary, but in "tooltip" format with no other information, and you have to wait 3 seconds.

This would be a great feature to be able to enable; anything that reduces having to navigate to another web page in favor of making information more transparently accessible, is a really good improvement in my book. Otherwise I can see myself jumping back and forth thinking "now what was that other issue again?" Ideally you could customize which fields would be displayed on a popup (e.g. like 1st 500 characters of description, last comment, etc.)