One big, pending task we've identified is making sure the class reference documentation is consistent with the changes we've made in Qt 5. Since this was originally written when widgets were the only way to do GUIs in Qt, some of the documentation is rather widget-centric in how it presents things. There might also be other content in there that is no longer correct, or less relevant, with the changes made in Qt 5.
The only way to fix this is really to go through the classes manually. We need to actually read the documentation for each class to make sure it still makes sense and presents Qt in a way that give people the information they need to make good decisions. If it is somehow unclear or seems stuck in the past, we need to update it with something better.
One example of such an update was in the QSyntaxHighlighter docs, where the description of the class referred to a constructor which had been removed in Qt 5, when the class was made independent of widgets. This reference was simply removed. Another example was the QTextDocument docs, which have been rephrased a little to identify the class as a general rich text layout engine, and not only as the back-end for QTextEdit, which is just one isolated use case in the context of Qt 5. Note, though, that for the majority of classes, there will not be any issues, but we won't know for sure until someone's read the documentation and verified it.
If you've been looking to get into developing on the Qt Project, this might be good time to get started. It's definitely a very useful way to get familiar with the different parts of Qt and the infrastructure around it, and the patches you'll be making are simple and low risk.
To organize the work, we've made a wiki-page. If you want to help out, please edit the page and replace "unassigned" with your name next to the class you're claiming to avoid duplicate work. If the class requires patching, please make sure the change is actually merged before you mark the class as "done".