The problem was basically this: every tank buys 2 massive armor items, 1 Warmogs, 1 FoN, 1 massive Damage item, 1 boots, and proceeds to be completely unkillable (because 500 armor, 300 MR, and 4000 hp means that nobody can touch you) even while you put out decent dps.
For example, Singed, Rammus, Galio, and Mumu would shred towers, and nobody could do anything. Even non-tanks like Garen, Nasus, Darius, Irelia, and Riven would ignore damage while dealing lots in return.
Magic damage has been persistently nerfed (overall) in the past, with increasing CDs, lower ratios, lower base damage, and higher costs. Auto-attacks always scale at 1:1 (or sometimes 2-3:1), are free, and have as little as a 0.4 second CD. The shortest spell CDs in the game are Karth's Q at 0.6 seconds, and Kog'Maw's Ult at 0.25 seconds; but Kog ult's cost doubles every time it's cast more than once every few seconds (i.e. 2x mana, 3x mana, 4x mana, 5x mana ad infinitum, and Kog has very little mana).
By forcing you to buy more than one item to get "lots of MR", and you only have to buy 2 items to make up for the old FoN: 2 Bulwarks give both bearers 90 bonus MR, or one Bulwark & one SV will give a single champ 110 bonus MR. So you can very easily run around with 250 MR, no runes, masteries, or abilities. You just can't have that and 3000 hp, and 300 AR, and do 600-1200 DPS.