"The "Katapult -> KRunner" transition represents, for me, many of the things that went wrong with KDE4: the new version is far worse than the previous."
Your comparison is flawed. Krunner is not a replacement for Katapult, it's a replacement for the old KDE Run-dialog. IIRC Katapult was a third-party app. If the developer of Katapult decided not to port his app to KDE4, that's his choice.
"- The new version is visually cluttered, while the older one was absolutely to the point. "
Um, Krunner looks very, very basic. It's just a textbox where you can enter text. Results are then displayed as icons and text. It couldn't really be any more uncluttered.
"- The new version has worse key binding defaults (alt-f2 against super-<space>) "
Krunner uses the old run-dialog shortcut, since that is what it replaces. That said, you can easily change it to something else (I use Alt+Space).