After 7 years of people discussing how the holy trinity of tank, healer, damage dealers is broken, after 7 years of tank shortages, after even the attempt to bribe people to tank failed, Blizzard is finally caving in. They are finally making a tank as easy to play as a damage dealer by making threat generation easier and automatic. To the point that "We expect the community to gradually stop using threat-tracking mods as players realize they don’t need them."
Now lots of people are going to discuss these changes in terms of "nerfing", "making WoW easier", etc. But the actual point of the changes is that it distributes the responsibility for success and failure more evenly in a group. If a large enough number of players would have actually liked to shoulder a higher responsibility, they had 7 years to express that by taking on one of the roles that tended to get all the blame in a group, tank or healer. It has been blindingly obvious that this wasn't the case, and that 80% to 90% of players preferred the less stressful, less responsible damage dealer role which was only foreseen for 60%. Even after handing out extra bribes for tanks, it was faster to find 3 damage dealers than 1 tank.
World of Warcraft always had the tools for a good group to share responsibility. Damage dealers always had the option to /assist, to throttle their damage, and in many cases help with crowd control or other aggro management abilities. People who only played in such good groups with highly skilled guild mates will probably fail to see the necessity of the announced changes. Unfortunately the reality of the Dungeon Finder and the pickup group was a very different one. The old model resulted in mediocre players of DPS classes to expect to be carried by overgeared tanks and good healers, throwing hissy fits if they happened to be grouped with a tank who wasn't geared and skilled any better if they were, or a healer who couldn't work miracles when the damage dealer's bad play resulted in drawing aggro. The old model channeled all the bad players into the damage dealer role.
In the end it was always the tank or healer who got blamed for a wipe, while the damage dealers measured their epeen in pure damage per second with zero regard for the survival of the group. As a social experiment of collaboration it was a complete failure, you can't have 2 people responsible for everybody surviving, and 3 responsible for "winning" by killing the mobs. You end up with 2 players either being blamed or being taking for granted, while the other 3 either do the blaming or congratulate themselves.
Note that while the threat changes principally affect tanks, there will be a corresponding strong effect on healers as well. If the tank holds aggro, a healer's role gets a lot easier than if the damage dealers constantly draw aggro and then blame the healer when they die.
The main disadvantage of the change is that it will move tactics back to where they were in Wrath, with the little restraint that damage dealers had in Cataclysm evaporating in an instant. It'll be AoE damage all the way, with DPS classes that aren't good at dealing area damage becoming less useful. But the alternative would have been a change which instead of lowering the responsibility of tanks and healers would have increased the responsibility of damage dealers, at which point group play would have become a niche activity. Making it easier for a tank to overcome the lack of tactical gameplay of the average damage dealer was probably the best option Blizzard had.