WPS 5.0 didn't support JSP 168 portlet API's and used to have IBM's Proprietary prtlet API's.
WebSphere Portlet 5.0.2 already supported JSR 168, learn more about it from this
article, b.t.w. by Stefan Hepper (IBM), one of the two specification leads of JSR 168.
The vendor specicific programming model by IBM is still supported in 5.1 (quite obvious that customers demand that), but you can for instance combine portlets of both APIs on one portal page.