Where can i find and clean up partially downloaded content?
The temporary location that stores downloading files varies between computers. Here's how to find it:


Start a rather large app store download
Open Activity monitor, and select the storedownloadd process. (storeagent for 10.9 and below)
Open the Inspector (press the "i" button, or ⌘+i)
Select the Open Files and Ports tab, and scroll to the bottom
Select the path that starts with /private/var/folders that contains com.apple.appstore from the beginning, up to and including com.apple.appstore 
Open Terminal and type open [paste path here] and hit return
From here, you can see all the temporary files the Mac App Store has made. You can get info on each one to see their sizes, and dispose of them accordingly.