dropping java 5 and 6 is a good thing imho. Supporting versions that Oracle itself no longer supports will only cause more of a headache... not only from a security standpoint, but from a features standpoint. Java 5 and 6 simply can't do some of the things Java 7 (and soon 8) can do.
For example, ARM blocks (Try with Resources) are great! But only available in j7+. When the time comes, Java 8 will support lambda expressions, etc. In Java 7 the GC was improved as well as the JVM itself had a lot of performance improvements. Many XML improvements as well (JAXP, JAXB, JAX-WS)
Full list of improvements and features: http://www.oracle.com/technetwork/java/javase/jdk7-relnotes-418459.html
the only drawback I can think of is if people try to run any of the igniterealtime.org software on systems where Oracle has dropped java support from (such as win 2000). However, we can simple say version X of software Y is the last Java 6 supported version, so those users live on that version until they upgrade OS's.
EDIT: Oh ya... almost forgot! JavaFX is increadibly awesome and makes creating rich GUI's very simple and easy (much unlike Swing). JavaFX can also run in the browser (similar-but-not-really to an applet). JavaFX only will work with J7+...
Message was edited by: Jason