I haven't noticed any killer features in KDE SC 4.4 and I've been running it since Beta 1. I've noticed a lot of subtle improvements. Things like app stacking and selection in the task bar seem much more responsive. All around, plasma looks subtly better and my favorite KDE apps seem to just keep getting better.
KSysGuard is really impressive and now has the ability to connect to remote hosts for monitoring. However, the biggest change is in the greater ecosystem. It seems all the external apps like Amarok, K3b, and digiKam are coming along to fruition.
Other than that, this is a smooth release and shows that the platform is starting to mature. I think the Summer release distros will be able to do a good job delivering a nice desktop experience based on KDE 4.4. I'll end with my obligatory "try KDE 4.4 if you had previous bad KDE4 experiences".