Here is a random assortment of problems I encountered:
- You can't configure size and appearance of the panel. This is a long-standing problem, but I thought it was fixed.
- In general, there don't appear to be all that many configuration options available to replicate various behaviors that I have become accustomed to.
- Rendering problems in the system tray; icons appear on top of each other.
- Various kicker widgets don't appear to be available for KDE 4 (yet?).
- Most applications don't copy their old settings, but it appears that some do?
- KMail doesn't interact well with encrypted IMAP accounts. I didn't try much beyond that, because I have no accounts that are not affected by that.
- Clicking on an RSS link in Konqueror doesn't link to Akregator anymore. Probably, various MIME type associations are missing.
- I couldn't get the environment to remember to open terminal windows maximized. Very annoying.
- I couldn't add any application icons to the panel. I'm not sure where on is supposed to store one's favorites now.
The good news is that the packaging appears to be really robust (I found one packaging bug with a file conflict) and the upgrade and downgrade works well (you have to use aptitude, as described in Ana's post; apt craps out completely).
So when KDE 4 is actually usable, Debian will be ready. But not for lenny; that would be suicide in my opinion.