As a WoW player, the most frustrating aspect right now is the difficulty curve of raiding past LFR. In the past few tiers, the tuning of normal modes has been all over the place as far as I can see from my own raiding skill and guild teams.
I understand that Flex raid is meant to be the raid setting for players that are of a skill between LFR and where the current normal modes are tuned for. However, this does have a few big disadvantages: yet another raid setting to tune for each tier, another lockout and further fragmenting the raiding community.
My suggestions to overcome those disadvantages are:
1. Keep LFR difficulty. Not everyone can raid to a set schedule or even desire to.
2. Replace Normal Mode with the Flex Mode (10 - 25 man).
3. Incorporate the "Determination" LFR wipe buffs into the Flex Mode raids (with the setting OFF by default!)
4. Allow raid leaders to set Determination buffing on or off per boss (like Heroic setting).
5. Change loot for Flex Mode to reduce ilvl by 4-6 points if a boss is killed while Determination buff has stacks.
6. Heroic Modes remain fixed at 10 or 25 man, no Determination buffs allowed.
The key here is to put the control over the difficulty in the hands of the PLAYERS, and also not create an extra headache of a separate lockout and another difficulty setting to tune each tier. There's a vast range of players above the LFR difficulty and hardcore heroic raiders in skill and playtime. Using these ideas would give those raiders complete control over how much difficulty they want and reward them accordingly.
TL;DR If Flex Mode raiding is implemented as announced earlier, WoW raiding will be spread across SIX difficulty modes with tuning needed for each (LFR, Flex, 10N, 25N, 10HM, 25HM). Take the Flex Mode in place of Normal reduces this to FOUR difficulty modes (LFR, Flex, 10 HM, 25 HM). Add Determination to Flex Mode and reward loot adjusted for whether it's on or off to further allow raiders to play at their preferred pace.