If go to 'help > What's this', and try to use it on any of the toolbar buttons, we simply get a red crossed circle on the cursor, indicating that the item is not clickable. I couldn't find any item that was clickable (I tried the sidebar and the page bar as well). In Kate, for example, the analogous help item works on toolbar buttons etc., so this seems to be some issue with Okular.
On Okular 1.3.3 it works only for Previous and Next buttons.
Not really a bug, just that we didn't feel any of those buttons really needed extra explanation. Which of the buttons you feel that would benefit from a what's this text?
Oh. I didn't realise it was intentional. Perhaps the 'what's this' menu item should be removed, then, if the application has no help text for any of the buttons?
I think, powerful tools which are not well known from other PDF readers would benefit. For example: Trim to selection