First and foremost it's a free-form note-taking application, not a Task-Manager, which is a huge difference. This alone makes the comparison to OmniFocus invalid, as OmniFocus is a full-blown Task-Manager, not a Note-Taking-Tool. 2Do is basically an application to manage arrays of text input (a list with strings). Of course it adds functionality like marking things as done etc., but at it's core it's just a fancy list.
As I stated in the first sentence, OneNote is a free-form note-taking app. That means that the program logic does not force you to use predetermined spaces (Views) to input data. Consequently you can just click anywhere on your "page" and start typing stuff. But not only that, as you can also cut, copy and paste from nearly anywhere and put it into your page as well. You could, for example, enter some text and press a button inside OneNote which lets you cut and paste anything thats visible on the screen. It's kinda hard to explain without going further into details, but OneNote is basically the nearest you can get to a piece of paper which you can obviously use to write down anything you want, anywhere you want (which is huge in order to arrange text, pictures etc. the way YOU want).
Besides Excel, OneNote is the best application Microsoft has ever written, and that's not because it's very complex, but it works flawlessly and very straight forward. Especially if you have a tablet-Pc, you'll LOVE OneNote!
If they didn't implemented the "free-form" aspect of the desktop app, I don't see "any" benefits besides Windows support though
