There are lots of suggestions for very good functionality, and there is a lot of very good functionality being put into XenForo, but there are a number of features that I personally won't use. Now my suggestion is to put core functionality into modules that can easily en downloaded and easily enabled/disabled.
This will allow the developers to add more of the functionality that users want, without bloating the core. And this will also allow for easy removal of features deemed unnecessary by different site owners.
But this could also easily be done with official add-ons and further improved with an automatic add-on installer.