Modifiers can prolong battles and make it harder for certain builds to kill some of the rarer monsters. Grinding Gear Games is also considering frustration when multiple similar mods are stacked and just keep extending the length of the fight and the longer sites just increase the challenge so much that either a person has to stay in for the long battle or run away.
The result of all of this is that the team will be patching and nerfing a wide number of modifiers, with an explanation to come tomorrow on their goals for the long list of changes. To start, in announcing the patch, GGG's Chris Wilson says:
We have nerfed many defensive modifiers. Our primary goal is so that when two similar defensive modifiers stack together, they don't make monsters incredibly slow to kill. In doing this, we have also generally reduced how punitive most defensive modifiers are towards specific builds (for example, builds that deal damage of a specific element).
The list that was published of the number of modifiers set to get nerfed was said to be subject to change, but even if there are tweaks to the list, these balance changes are going to be felt. There are 22 categories listed, with individual modifiers getting one or more changes. These include things ranging from reduced damage reductions, as on Toxic, which drops from 40% to 25% Chaos Resistance against DoT, eliminations of some damage resistances, Deadeye losing both the 60% damage reduction when recently hit and the 200% increased Evasion under the same criteria, and much more.