I did a fresh install with a minimal X server in order to do an install of Qt5/KDEF5/Plasma 5 only then installed them via the standard 13.2 repo (with the plasma5-session package/pattern). I don't know if it matters but I've left the compositor enabled with the default OpenGL 2.0/GLX settings and disabled all Desktop Effects for now, at least.
The Plasma UI (UX?) is scaled way too large in a lot of respects. Especially icons in the Plasma UI, not the desktop icons however. The 'Add widgets' thing is way too large to be usable. There are a few things that look out of place too. Here are some screenshot examples.



I'm running 13.2 x64. Here's some relevant info:

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 disconnected (normal left inverted right x axis y axis)
HDMI-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 160mm x 90mm
   1920x1080     60.00*+  59.94    30.00    24.00    29.97    23.98  
   1920x1080i    60.00    59.94  
   1600x1200     60.00  
   1280x1024     60.02  
   1360x768      60.02  
   1280x720      60.00    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
DVI-0 disconnected (normal left inverted right x axis y axis)
lspci | grep VGA
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Pitcairn XT [Radeon HD 7870 GHz Edition]
find /dev -group video
server glx vendor string: SGI
client glx vendor string: Mesa Project and SGI
OpenGL vendor string: X.Org
Any suggestions on how to get it to a "normal" state?

P.S. I want to post this over on the KDE Forums as well but it's impossible to create a user account ("KDE Identity") right now. :/