Here's the thing, Apple really made a good decision when they chose to use iPhone OS on the tablet, think about it this way, iPhone OS is designed to run on mobile devices, it's optimized to do so, with lower power requirements than a full blows desktop/laptop OS, which is an advantage over other tablets since most others usually run Windows.
As to its advantage over an iPod touch? Well, as some people pointed out, screen size really DOES make a difference. It's really hard to do good presentation work, or flowcharts, image editing, movie editing or what-have-you on a 4 inch screen, the extra room makes these tasks possible, and plus, this thing CAN do just about everything a computer can do, yes it can't use flash, and this is a drawback, but i believe that all the extra screen space allows for way more possibilities than an iPod/iPhone, so it works out.