Recently, i have found deathcap to be better for tanks like nunu, mord, and amumu than it is for ap bursts. there are other items that simply do deathcaps job better, and then some. For instance, i would usually build deathcap if someone was building lots of health because i would need to do more damage to eat through their health. But with the addition of items like liandry's torment and the changes to deathfire grasp, deathcap is no longer the best choice for these situations. In fact, i find that the only time that deathcap is a good buy is if you are mostly tank with a tiny bit of ap from items like rylais, abyssal sceptor, and/or liandry's torment. It is great for ap tanks because of their high ap ratios and massive survival ability. But again, for extremely squishy burst champions, there are better items that give similar amounts of AP and give much more situational utility. i really loved building deathcap on veigar since his q gives him lots of AP to scale up. But since building warmogs is one of the easiest ways to counter cooldown burst like veigar, liandry's torment and deathfire are just so much better. So to bring my rant to a directed point. Was deathcap intended to be an AP tank item?

Rabadon's is not a "tank" item, it has no tank stats. However with Season 3 itemization, tanks can finally build AP and feel good about it. The three big AP items are all within 200 gp item difference.

Rabadon's offers 120 AP + 30% AP multiplier =150 AP upon buying as first AP item

DFG offers 100 AP and the active eats 15% of current target's health and they receive 20% more damage. Compare DFG to Rabadon's:
-----DFG deals so much more damage to a single target. IMO DFG is the best huge AP item to build first for damage.

Zhonya's is 100 AP 45 AR and 2 second invulnerability. This item is situational.

It all depends on the situation. Of all the AP mids, I play Annie the most. Generally I build Sorc boots first, then either Rabadon's second or Liandry's Torment. I think I might build DFG first from now on.

Generally the difference between burst-casters and tanky-derps is the actual AP ratios - burst casters tend to have better ratios than supports or tanks more often than not, so grabbing plenty of AP on them is generally a more rewarding choice.

Alternatively, magic penetration and CDR tend to be better at increasing the damage and utility of tank and support abilities, so you're generally better off with grabbing magic penetration on actual tanks and supports. Because there AP scaling tends to be a little worse. Also, items like Abyssal Scepter and Torment actually grant survivability stats so you can tank or support without evaporating, which is handy.

If you go with a slew of full-tank items like Warmog's, Spirit Visage, then Deathcap that works fairly well though. You just end up advancing through the game like a Carry were you hit a substantial power spike after each item rather than gradually advancing as the game goes on.

if amumu or malphite happens to get an early lead, they will finish the tank items they need so fast that they can start building damage. I have seen time and time again how high amumu AP ratio's are, his ultimate is hard to beat or even escape in any situation if he has liandry's, rylais, deathcap and sunfire cape. I really love playing amumu, but i will be the first to say that something about that situation needs to be tweaked. Amumu is not balanced. Even if amumu doesnt get an early lead. he can just keep farming and once his ultimate is lvl 3, it will ALWAYS win teamfights unless he and his team are ridiculously outclassed.

I'm all for tanks building ap when they should be building resists. Means I get to catch up to them. Amumu is kind of a special case though. You can't base a claim on an entire class of champion on just him. =\ If I saw a Leona or Nautilus with Deathcap, I'd kinda laugh, really.

Amumu and Malphite aren't really proper representatives of Tanks as a whole, as they're more or less the best ones in the game - so they'll generally put any mage that isn't Karthus, Oriana, or Anivia to shame by comparison.

Beyond that anyone with AP ratios at all and an end-game build with plenty of AP will beat you to a bloody pulp and prevent you from escaping, the fact tanks can do it isn't terribly surprising. That's the reason gold remains incredibly relevant in the outcome of LoL, and the reason getting an early lead can cause you to snowball out of control throughout the rest of a game - which should cause you to win fights, and is something that can be said of every champion. So the fact 4 items allow Amumu to do that isn't impressive or unusual in any way. Amumu's a really strong pick more because he can win team fights through the utility and potency of his abilities alone without any items at all, which is the category most champions with a heavy focus on side-effects over flat damage tend to fall into. Amumu has a stun, gap-closer, AoE stun, free damage reduction, and %max health damage to remain relevant even when other champions would be shut down.

Your info is out of date, man.
Deathcap has been 120 + 25% even since the new jungle items were added.
Preseason 3 patch notes

This last patch a few days ago made these changes:

DFG is now 120 AP

Zhonya's is now 120 AP and builds from a new item

Patch 3.01 Notes

I would agree that the current Deathcap needs help.
How about this:
Rabadon's Deathcap Cost: 3800 (905 + Needlessly Large Rod + Blasting Wand + Amplifying Tome)
+170 Ability Power
UNIQUE Passive: Increases Ability Power by 25%.

Naw. Rabaddon's Deathcap is fine. Making it too expensive would make it more situational. People get it more for its passive rather than its base AP. The increase in the the AP of Deathfire Grasp and Zhonya's just made it even more attractive, since now the +25% makes them all bring in 150 AP each with the Deathcap.

You get Rabadon's to scale your other AP items, just like you get PD to scale your other AD/crit items. If it is your only AP item, you are doing it wrong. If you get fed on Amumu or Malphite, you get Haunting Guise/Sunfire/Abyssal to help with your base damages or you get Zhonyas to outplay people when they focus you.