I upgraded to KDE-4.5 only when I upgraded my slackware distro to 13.37. This version is already quite dated, it's usable, but I still miss some kde-3.x features that still do not exist (that I am aware of) in KDE-4.
Sounds silly, but I really miss kpanel the most. It is the best virtual desktop manager I have used (from 3.5.x series), showing thumbnails of all applications (can recognize by sight), proper desktop backgrounds, and ability to move windows from desktop to desktop from inside the panel itself. KDE's (at least as of 4.5.5) panel app doesn't let me move windows, and it doesn't show image pixmaps of wallpapers or applications. Usually the text of a window doesn't fit, and it's quite unusable to be able to tell one window from another.
I also used to use Kmail heavily, but the new one seems so much more bloated, requiring SQL (is that akonadi?). Don't know why I need an SQL server running to read my email off an IMAP server (messages stored on the IMAP server, not "fetched" to the local KMAIL app).
I also don't quite get the Activities and Plasma stuff, would prefer to just have a plain virtual desktop like KDE-3.5.10.
I've thought about Trinity, but Slackware-13.37 isn't officially supported (despite being more than a year old)... maybe it would work, but I haven't had time to try yet. Probably makes more sense to try and upgrade slackware-13.37 from KDE-4.5.5 to a newer version.
I also must be one of those "older" folks who prefers the standard drop down cascaded menus. I don't need a "start" menu that isn't big enough to show me everything, and require scrolling a menu (what a stupid concept, thanks Windows7), or requires typing to find what I want. At least KDE can still be customized better than Windows7 can be.
I also prefer the older Konqueror file manager over Dolphin. Just from a user "experience", there seems to be more visual polish on KDE-4 but less functionality than KDE-3.5, in my opinion.
Is there a Subversion plugin for Konqueror or Dolphin yet (TortoiseSVN equivalent)? That's one thing that is sorely lacking from KDE-3.5.x series since SVN was only taking off about then.