Here are the keys to Norfair:
You lose nana mobility and nana locking.
Nana locking isn't that big a deal though because against really good people it is rare you will land the locks, its usually one nana hit > dash attack > air attack or one nana hit > grab > look for follow up nana hit/dash attack/edge guard.
On Norfair you can basically set up camp on whatever the lowest safe platform is and the opponent has to approach from the crappiest of positions (usually just straight above). Then, on top of this, your Bair when the opponent is over 50%, Fair when they are over 30%, fthrow/bthrow when they are over 30-40%, have great trajectories for sending people into lava. (compare this to MK where you basically have to be over 50-60% for anything to have a reasonable chance of sending you into the lava before you recover). Not only that but Diddy is quick enough to do lava combo's, which are fun as hell when you land them.
Another note is that, while Diddy has an amazing recover as it is, on Norfair Diddy is basically UNGIMPABLE, its crazy, your Side-B is so sexy on this stage for moving between platforms, for recovering, for everything. Then even if you can't get back to the stage by humping you can still barrel and choose one of three edges, making it so the opponent can't touch you.
Finally, Norfair is like PTAD in the respect that if you know the stage you will never (or I guess very rarely) get killed by it, while you can almost assume an opponent who doesn't know the stage will get gimped by the stage at least once. Even without the gimp KO's though, intentional lava KO's aid Diddy in his weakest attribute: KO power.