NOTHING. Good grief. What you aren't getting here, is that 99% of iTunes users have never changed the import settings, ever. They wouldn't know the first thing about changing it, or why.
The only thing the masses understand is low quality vs. "high" quality, and even that is barely understood by most.
In short, if you use "high quality" iTunes plus, you are getting nothing better than original iTunes. 128 or 256. You have a choice between crap and crap.
256/Plus is "better", but to say its high quality is inaccurate.
What a confused response.