Is Apple TV intelligent enough to stream directly from a NAS?
To answer the op's exact question, "No, ATV is not intelligent enough to stream directly from a NAS".  Others have pointed out some options, like using ATV2+Plex, or PlexConnect (which is VERY difficult to setup).

Let me thrown my $0.02 in by saying that unless you're running your iMac over wi-fi, this will work fine.  Even with my older 802.11g network, my Mac mini (hooked up to Ethernet and thus hooked up "closely" to the NAS) could pull the files off the NAS and stream them (over wifi) to the ATV.  No issues at all.  I'm on 802.11n now, and it's handling 1080p streams no problem (don't remember if I tried that on the "g" wifi).  In fact, with the Mac on Ethernet, it can download new videos from the iTunes Store while serving up content from the NAS to the ATV.  It's really not a lot of bandwidth.  You can check in the Activity Monitor.  I'm always surprised by how little bandwidth it uses.

However, if your Mac is on wifi too, I could see that there could be some problems, but a "n" (or "ac") grade wifi should be able to handle it.