If you're trying to drag and drop an icon into an existing folder, you need to move quickly and decisively. I have found that if you drag an icon slowly toward a folder, the folder will think you're trying to move it out of the way. But if you drag it quickly onto the top of the folder, it'll stay still and accept it.
Edit--In fact, I just found the exact part of the Nexus 7 manual that covers this, on pages 17-18:
"To bump another icon out of the way, slide slowly into it.
To combine two icons in a folder, slide one quickly over the other." [bold is mine]
Aren't manuals neat?:cool: