As you probably know already, Pentaho offers two versions of Kettle, an open source Community Edition (CE) and an Enterprise Edition (EE) that contains proprietary pieces of software. On the Pentaho website you can learn about the differences between CE and EE in general (
http://www.pentaho.com/products/enterprise/enterprise_comparison.php), and as you can see from the site there's a lot to be gained from taking an Enterprise subscription.
One thing to keep in mind about the Enterprise Edition is that you sign up for a yearly subscription, which is different than a software license. Most software licenses include the right to use the software for an infinite amount of time, and even if you stop paying for support, you can still keep using the software. A subscription works differently. Each year the subscription needs to be renewed and a new license key with an expiration date set at the current date plus one year will be issued. This means that when you decide to terminate the subscription, the software will stop working as soon as the license key expires. However, with Kettle there could be a smooth downgrade path from the Enterprise to the Community Edition; as long as you don't use any of the EE-only steps (see following), all jobs and transformations built using the Enterprise Edition will run unchanged on the Community Edition as well.
Until version 4 of Kettle, there weren't any significant differences in functionality between ...