Quote from the front page:
1. "The new Flex loot model will work similar to how Master Loot does today"
2. "Loot will scale based on the number of players, giving you a higher chance for another piece of loot the more players you have. This ensures there are no optimal breakpoints that would limit raid size to get more loot."
3. "Each raid difficulty has its own lockout. They will be loot based lockouts, allowing you to kill the same boss multiple times per week and only get loot once a week."
Possible exploiting scenario:
Imagine a hardcore top-tier raiding guild. They form a 25-man raid, consisting of 24 alts and 1 main character. They clear the instance, with this 1 main character getting all the loot (which is 5 random items PER boss PER difficulty in 25-man). Next, they form a new raid, consisting of 1 main character, who already got his loot + 23 alts + 1 new main character, and clear the instance again. Repeat these steps for as many times as you have raiders.
If you think top tier guilds won't do that, think again
Yes, this was possible earlier, but it was way harder, fairly limited, and dependent on the number of your alts. With the new system, you only need maximum 1 alt per raider to pull it off and the rewards are way higher.
Obviously, I am talking about exploiting this during the first week, when no mythic raids are open yet.