The main differences are in what standards are supported. See these links: Standard Compliance Support for WLS 5.1 Standard Compliance Support for WLS 6.1 The other highly noticeable difference is in the Management Console. WLS 5.1 had a heavy Swing-based console. WLS 6.1 has a thin Web-based console. WLS 6.1 also has better clustering capabilities and includes support for Web Services out of the box.
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