I really like this CMS and I think it's pretty intuitive to use. But there is a problem, mainly with the modules: They lack end user CMS integration.
* The forum module uses BB code for formatting, which works, but it makes the DB contain different content formats where it's not really required. I think a html-editor control should be used instead, kind of the one that is in the CMS but a bit more restrictive.
* The blog module, which also uses BB code, should do the same for the editing form, but should also allow using the CMS including the WYSIWYG html editor. To make that practical when dealing with permissions and such, it would be great if it was possible to make a certain user able to use the CMS to edit his own toplevel BlogHolder and all the BlogEntries below it in the hierarchy. Maybe even make the BlogHolder the root element for that user.
I don't know how hard these things are to do, but I'm guessing that it will be a lot of work for the second point. Anyways, I really think that these would be features that would take SilverStripe to the next level.