A few things I've observed in testing out KDE 4.5.4 on Win XP 64-bit (testing the 3 different compilers - so far the mingw4 compiler has produced the best results):

The activity manager isn't working (any compiler):
Whenever I open the activities dialog, it doesn't show any activities, and whenever I try switching activities (setting mouse-scroll action to switch activity and using that) plasma-desktop crashes. I tried starting the activity manager from system settings, but that didn't do anything.

The add widgets and the panel settings dialogs disappear whenever a popup shows up or any other desktop element is focused – annoying as the popup description ends up triggering the dialog closing.

The desktop settings panels are quite slow when using the MS VS compilers (opening up the settings, and selecting and applying wallpapers primarily).

Several plasmoids are missing, including: preview, notes, rss (any of the sort).
No KDE PIM - only the pim libs are installed, no kontact, kmail, akregator, etc.