If you take a look, you'll notice that many of the really hard problems that require hard work and thinking are still there.
Take a look at the "most hated bugs" (http://tinyurl.com/2zkqr6) and search for the bugs with smaller numbers. They have been there for many years because are hard to solve, for example #70063 for which I constantly receive activity in the inbox because of being subscribed.
There's also "most wanted features" (http://tinyurl.com/l5fg7m), with bugs from 2001 in top 10. I receive some activity from there too.
That's not to say I am comparing KDE with Xorg's 255 keycodes limitation (the first that comes mind now). Far from me. In many respects KDE is a fast moving project.
Otherwise, the feel good air is there, and I switched to KDE 4.2 as soon as it made it to Debian and didn't look back too much.