The question on everyone's lips this week is "What did Rockstar change to move Manhunt 2 from AO to M?". Well in the latest review by IGN they reveal all, the game hasn't change majorly at all in fact:

In the M-rated version, Rockstar has added both an extreme blur effect and in most cases darkened the graphics so that it is nearly impossible to make any sense of what is going on.

GameRadar has you cover from their on:

With the exception of the removal of the infamous testicle/vertebrae displacement, we're told all of these executions will be intact come late October... albeit somewhat altered. Now, upon entering the execution scenes, there's a newly added visual layer of weirdness. Everything goes all Christmasy, as if through a dark red and green filter, with pulsating motion blurs of blinding white. You still get an idea of the mayhem occurring on screen, but it's far less obvious and over the top as before. At best, the ambiguity enhances the disturbingly macabre motif. At worst, it makes watching the exhilaratingly barbaric execution scenes a little like trying catch a glimpse of a scrambled, late-night tit on a HBO.

So as you can see, apart from the obvious remove all the testicle removal scene and the new 'visual layer' to prevent direct viewing of executions the game hasn't been changed dramaticaly. However word is from GameSpot that Toronto has added to the dialog between Leo and Danny, whether this is just for the Wii version or not is unknown as of yet (as Toronto is working on the Wii version only):

This time around, the developers at Rockstar Toronto have added more dialogue between Danny and Leo, but the core mechanics are still the same.

More information as it becomes avaliable on the modification of the game. But so far, it looks as though the game will be just as good, if not better then before.