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While we opted to not require servlet 3 for wicket 6, it has bitten us a couple of times solving issues in wicket 6. Servlet 3 has been out for ages, so we should move with the times.
Improved tag manipulation
Using attribute modifiers it is difficult to modify tags in proper ways such that existing classes on a 'class' attribute remain, while adding/removing other tags. A DSL for manipulating the class value of a component would be really great.
Make transitions possible/easy when replacing components using Ajax/Push
Currently we replace the markup of components, which works for updating the components, but it doesn't provide an easy way to animate the outgoing and incoming markup using CSS transitions.