EDIT: So this rapidly devolves into a QQ thread, for which I appologize; that was not my original intent. Constructive community feedback would be appreciated on the issue.
Riot's stated goal in the season 3 itemization changes was to "allow players the possibility to create more non-standard builds, and to change their build to account for different play styles, and in response to different teams."
They've hit the mark pretty well for Adcs, and AD bruisers; while there are still certain core items which everyone builds at some point or other, the build order is flexible depending upon what one needs, and allows for varied gameplay. This is helped along by many low cost items which combine into higher teir items, as well as the fact that they can freely itemize tanky or high damage output, and in the end get both stats from their builds.
On the other hand AP carries (not bruisers like akali/eve/kat/rumble) now have to build pure glass cannon in order to do any sort of damage, and many of the high damage items do not build out of cheap, easy to aquire items (i.e. Deathcap/DFG/Zhonyas are the core of any build, and Needlessly Large rod is between a third to twice as expensive as its AD counterparts (note: the item is more comparable to an item like the brutalizer than BFS since the secondary item in the BFS upgrades all provide more utility than say amplifying tome, and the upgrade from Needless to Deathcap/DFG/Zhonyas is more expensive, and pays off less than upgrading to say an inf edge or Bt).
This means that while ADcs and AD/AP bruisers have had their damage and sustain buffed considerably from season 2 to 3, AP carries have had their damage nerfed to unsustainable levels. What's the point of having a burst/cc APc when you can get better, quicker burst from an AD bruiser, who also gets free sustain from the build (WoTA/revolver don't give much, and sidetrack from actual damage output).
APs can still build glass cannon dps. by rushing a tear of the goddess, then Deathcap, they can achieve comparable AP to what RoA and Deathcap gave last season. The problem is that this only works for carries who can sit comfortably behind their entire team, and with the buffs to AD bruisers/assassins this is impossible. Also, the lack of tankiness/sustain in the build means that in a 1v1 fight with anyone other than the enemy support an AP carry will lose.
So, either the meta will shift towards APc bot w/ support, which is non-feasible since ADcs need the extra sustain early game and currently outscale APs. Or they will remain forever benched, as they have been for the last couple months.
Riot: We need some additional build options aimed at APcs. I realize this will be hard to balance in light of the efficacy which the S3 items have lent to AP bruiser champs such as Kat/Rumble, however considering the nerf to deathcap, perhaps you could redistribute the AP better? Buff back Athene's AP ratios, or make Haunting Guise upgrade into a sustain item aimed towards non-bruisers. Make it so that the secondary item in the three big AP goals (Deathcap/DFG) actually gives comparable damage/sustain to AD counterparts. Or just generally balance AP items so they're really good early game, but fall off late; make them the new bruiser -- I personally wouldn't enjoy this, but at least it would make them viable in some role again.