Inspired by this thread: http://brilliantgameologists.com/boards/index.php?topic=2031.0
New 4e statistic: Spell Power (or, if you prefer, Exploit Power or Prayer Power). Spell Power is determined after hitting with a power that has an effect that a save can end. The sum result of Spell Power is then applied as a penalty to saving throws the target makes against the effects of the power. Like other attributes, Spell Power has specified bonuses such as Feat, Item, and Power bonuses. For example, Spell Focus provides a +2 Feat bonus to Spell Power, the Wizard's Orb of Imposition ability applies a Power bonus to Spell Power, and a Phrenic Crown provides an Item bonus to Spell Power. Bonuses of the same type don't stack.
Furthermore, there are some more STRONGLY recommended houserules intended to chop off the end of the power curve:
Cunning Weapon: This item's property does not improve at levels 18, 23, or 28.
Earthroot Staff: There is no level 3 version of this item. This item's Property only ever applies a +2 item bonus to Spell Power with powers that impose the listed conditions.
Phrenic Crown: There is no level 7 or 27 version of this item. This item's Property only ever applies a +1 item bonus to Spell Power.
Skull Mask: This item only provides a +2 Item bonus to Spell Power for yourself. It does not impose a penalty on enemies otherwise.
If I missed any other new items in Adventurer's Vault with outrageous effects, please let me know. AFAIK most of the book is only nifty (which is nifty, but not particularly powerful).
Note to WotC: Learn your own damn system! You made even small penalties to saving throws effectively scale with level by eliminating just about every bonus and penalty to them and setting the DC to 10 at all times, so don't make ANY more items with scaling saving throw penalties at higher levels EVER AGAIN! It's bad for the game!