Apart from the argument of Wicket\'s simplicity (that is, Wicket is a simpler system IMHO) and GWT\'s responsiveness in the client (GWT\'s client side state and JavaScript -
There are few other advantages of wicket over GWT that I find.
GWT will not work with a browser where javascript has been disabled . (this is rare though). Wicket always falls back to normal http requests if javascript is not available.
GWT applications are one page application this makes bookmarking and using browser tabs abit hard. With wicket you choose to create one page application or multiple page applications. You can make the pages bookmarkable if you wish.
In GWT, creating your own components is not always easy. In wicket, since you are working with raw html, css and even javascript making custom components is very flexible. You can even wrap an existing jquery or dojo component very easily.
Since GWT involves compiling java to javascript, you can only use the java classes that have been emulated by the GWT Compiler. This can be limiting. Wicket is a server side framework and you can use all the java you want.
Working with CSS and web designers is much easier with wicket that GWT.