Mostly full control over the driver.
Example: some third party cards (powered by ati, ect) don't include the rotate option in the display options. It's still there, however, and irotate can access it. Also in terms of future development, irotate allows you to rotate the screen via the command line. Simply call "irotate.exe /1:rotate=90" to rotate monitor one, 90 degrees. That's another thing.... for dual monitor setups, it only rotates the mointors you wish to rotate. This allows fe and utility developers to utilize on the fly rotating. (Rotating via hotkeys without letting a program know can cause display errors, so it's best to let the program itself call the rotation when it's ready.)
Also in general, on the ati side of things, I've found it to be more stable.
So there aren't any new features, rather irotate isn't buggy like nvidia and ati's driver interfaces.