Here are some of the features that may lead you to consider struts2 :
1, Action based framework
2, Mature with a vibrant developer and user community
3, Annotation and XML configuration options
4, POJO-based actions that are easy to test
5, Spring,SiteMesh and Tiles integration
6, OGNL expression language integeration
7, Themes based tag libraries and Ajax tags
8, Multiple view options(Jsp,Freemarker,Velocity and XSLT)
9, Plug-ins to extend and modify framework features
difference
struts2 is an action based MVC web framework
JSF,Wicket,Tapestry are component based frameworks
history
in the beginning of 2005, Jesse James Garrett coin Ajax(Asynchronous JavaScript and Xml)