Given that a lot of KDE3 apps are not just ported to KDE4 but replacement projects have been created, the upgrade is not trivial. I always declare myself as somebody who is a minimal KDE user saying I mostly just use konsole, but that's not really correct. From time to time I use the following apps as well:
And that really doesn't belong to here, but I still use gtk-based apps for browsing and picture viewing (firefox and gqview).
I hope this post will be able to serve as a good personal checklist when we merge kde4 to current. ;)