General lack of polish. This is my #1 complaint about KDE, and it's everywhere.
Personally, I disagree that's it's everywhere. Yes, it's here and there and usually in applications that are either very new (=incomplete in general) or remnants of KDE3. But there is an initiative that works on ironing them out:
Crashiness. Sometimes, daemons decide to crash randomly. Occasionally, the compositor goes crazy and locks up the entire desktop.
Never experienced that (openSUSE with official NVidia drivers here). Either these are those infamous (K)ubuntu-specific bugs or they are related to the GPU driver.
- Insane defaults. Preferences are nice, but they need to be set to reasonable values by default. For example, there are *way* too many global key bindings by default, the eye candy is set to an annoyingly high level by default, single-click select in file dialogs contradicts every other desktop, the default panel is huge, and a whole ton of other things.
Global key shortcuts may be a case for Extra Mile. The others are things the distributor should decide for his user base.
- No good system monitor widget. GNOME 2.x had an awesome panel widget that would display CPU, network, and memory; it even displayed I/O wait CPU time in a different color, which was awesome.
There are several 3rd party ones available. No idea how good they are because system monitors are useless bling for teenage boys IMO.
- The cashew. It makes no sense, and you can't get rid of it.
It makes sense and you can remove it. One of Plasma's goals is not to rely on right clicks which is why common users should not be able to completely break their desktop. It's not a GNOME-like "Settings confuse users" thing but imagine a user who uses a touchscreen and first turns off the cashew and then accidentally removes the task bar. Without the ability to right click the desktop would be completely broken until a mouse is attached.
As for removing it: It's possible and has always been possible. Technically the cashew is just another Plasma widget. Personally I just move it behind a panel but there are tools to regulate the cashew's opacity or even remove it completely, eg http://kde-look.org/content/show.php/Py-Cashew?content=147892