I also run KDE with a 20 Gb root partition. I am now using 30%. I doubt I will ever get to 100%.
I'm not very familiar with Nepomuk(some kind of indexing service?) but in systems settings the main enable checkbox is unchecked, by default I believe. I haven't touched it and I don't want it.
i5 3570k, ASRock z77 Extreme 4, 8 Gb Ripjaws 1600, Antec 430w psu, HVR 1600 tv tuner, custom case- marble top, oak face. Carver & DCM Time Window sound system. Mint 14 KDE.