"Flex" sounds too kinky and "mythic" is unnecessary.
They should only have normal (that flexes) and heroic. And LFR for the lore and nab gear.
Besides, flex+normal right now are a joke coexisting, flex is obviously not supposed to be "easy", or at least normal isn't that much harder. There is no reason for having both.
Besides the #1 problem of especially 10mans were missing one or two people and being screwed. Now it's best for those people to aim for 12-13 people and call it a day.
Pretty sure they already have a solution for what you are criticizing. Right now they have 4 raiding difficulties, LFR, Flex, Normal, and Heroic. In WoD they are making it a bit less confusing with LFR, Normal, Heroic, and Mythic, with normal and heroic both being flexible but with separate difficulty levels.
I'm not sure you understand the meaning of the word kinky. This topic has been done to death. I'm not sure it needed a new thread to express unoriginal opinions.
I'm not sure why you feel the need to create new threads every single day complaining about the smallest things in regards to World of Warcraft but yes, this is a topic that has been covered a few dozen times. There are threads in our Raids & Dungeons and Warlords of Draenor forums that cover this topic.