Well, the reason KDE 4.0 was to buggy was exactly "because" KDE had contributors. Porting from Qt3 to Qt4 was a lot of work. Some volunteers completed that much much earlier than others but had to wait for others parts of the project to ship. More complex parts weren't ready yet, and as such, some contributors were losing interest. So KDE had to ship it at some point to make sure the train keeps moving.

It was a combination of the massive, massive hype train that was generated by KDE + distro's not listening to KDE's ask for 4.0 and earlier versions not being shipped that caused a massive backlash. KDE learned from the mistakes and the 4 to 5 transition and 5 to 6 transition which is in the works now are both avoiding those mistakes. But KDE did fix those bugs. Within a few releases, KDE 4 was quite usable and stable, but the damage was done.

In my opinion KDE hasn't really recovered the bad image caused by that despite the fact that its in a much better position now.