I just picked an OTG cable and experienced the same issue - just plugging a USB flash into my Nexus 7 yields no joy. Now for the interesting thing, I tried the same thing with my Samsung S3 running 4.0.4 and it immediately recognized my flash and let me start playing tunes from it.
Any ideas what extra element has Samsung loaded to allow this? This is the way things are supposed to work.
Samsung and just about every other vendor added back the missing automount support so it works they way it should, without needing to root.