Besides the release of JIRA 6.0 RC 1, Atlassian created a short trailer about JIRA 6. This one gives you the possibility to have a look at the new UI. The new style of JIRA 6 is very minimalistic and is similar to Stash. Besides that, JIRA 6 features mobile optimized views for your smartphone and many usability improvements.
Have a look at the detailed release notes for an overview of the new features implemented in JIRA 6.