What are main features of Java 1.6 ?

Java 1.6 Updates

Performance Improvements

Faster jar File Creation

The fix of a long standing bug related to jar file creation has greatly improved creation time. For example, for a given jar file, it is possible that you might see a creation time improvement in the range of 20 percent. (Refer to 6496274.)

Garbage collection improvements

Updated Client JVM heap configuration 

In the Client JVM, the default Java heap configuration has been modified to improve the performance of today's rich client applications. Initial and maximum heap sizes are larger and settings related to generational garbage collection are better tuned.

The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte. 

Runtime Performance Improvements for UI Applications

Improved performance of applications using translucent windows. (Refer to 6794764.)

Better performance and smaller memory consumption by text rasterizer. (Refer to 6891557 and 6891551.)

Faster processing of PNG images. (Refer to 6549882.)

Ability to Read Larger .zip Files

security 

New JNLP Extension Security Dialog

A new security dialog has been introduced in 6u18 for installing signed Java extensions on a user's system. Java extensions are components described in JNLP files that are typically intended to be used by a large number of applications and applets.

The new security dialog is triggered by an application that references the Java extension in its JNLP file (by including it in the resources element). The security dialog will ask the user if they would like to install the extension. Once the extension is installed, it can be referenced by other applications without the need for asking the user's permission again (as long as it is the same extension from the same codebase).