CONTEXT:
"We wanted mages to have less AP overall, do less damage and be, for the most part, less effective. On the plus side we're giving them CDR on ever item, but---jokes on you---that CDR is a shared passive that does not stack. Okay, but you are going to get more mana. Sidenote: "Mana" refers to a resource that is used to gate the amount of spells that champions can cast, each spell costs a certain amount of this "mana" and once you have 0, you cannot cast anymore spells... I added this sidenote because "mana" is an irrelevant stat in League because most champions never run out of it... okay back to the context...
We felt like mages are just so overwhelmingly awesome right now that their damage needed to be roped in a bit. We are giving a buff to Rabadons Death Cap, but this doesn't really offset the overall nerfs to the amount of ability power on other items, additionally we have heard what the community has to say about Rabadons being a rather gold inefficient item and we are---as usual---ignoring you.
TL / DR: Change for the sake of change, that's your context"