Magtheridon was a bit too hard for most people and so Blizzard have nerfed his adds to have about 120K HP less eahc.

Reprorts have it that although Mags himself hits harder now, the adds have less HP (120k less, according to one poster). Magtheridon was significantly harder than Gruul, who he was in theory meant to be on par with; hopefully this, as well as the Grull fixes, help make beginning BC raiding a little easier. I mean, I'm all for having to work for your epics, but I think most of you will agree that the beginning of the new endgame was a little too much like a brick wall

Why is no one talking about trying to down the 25 man instance boss Magtheridon yet?

Well as someone from the very well known Death and Taxes guild says, they tried him a few times and got owned. Either he is too gear dependant (i.e. you need phat epix lewtz to kill him), or Blizzard haven't finished tuning him yet.