As I see it RiotNome, the problem is not just the defensive items but all items in general. Some items in dominion are just too powerful due to their SR nature. Trinity Force, Infinity Edge, etc. They need new "toned down" versions like Madreds Bloodrazor becoming Kitaes Bloodrazor. Casters could also do with that selfish strong spellvamp item so they actually stand a chance in fights endgame without having to be tanky dps like Kassadin and Ryze or true damage like Ahri.
The reason that people have to go tank or bust is because of the naturally tanky champions such as jax that can one shot you the second they get one of their 3.5K items. The power creep overall is too high, both defensive and offensive items need nerfed dominion versions IMO, but mainly the offensive items due to the innate scaling granted by either the champions passives (Kog Maw, Vayne, Etc) or Last Whisper/Lifesteal.
IMO no Item in dominion should cost over 3K. That way you can balance power creep by having less power to creep with. Kitaes bloodrazor was a good start, the only reason that is seldom used is because no other SR item has a nerfed dominion version. This is why the dominion only items are seldom used except for Hextech Sweeper, the SR items are too strong in comparison.
@Sauron while raising the CDR cap is nice in theory, it'll not do much since mages would have to find some way to get even higher cdr without loosing any stats they need, and the mana required to sustain it. It's just not a solution and would just proceed to make tanky casters that can abuse Tears of the Godess such as Yorrick, Urgot and Ryze even worse than they are now. The solution needs to be something that effects AP casters only and energy/rage users, otherwise we are going to see alot of ghostblade/brutilizer CDR boot AD's destroy everything. So to that end the Crystal Scar aura needs to be adjusted, say a flat AP % boost or a % of your current AP is added as flat magic pen or spellvamp. Stuff like that. It must not effect AD's or tanks at all.