This is a fantastic idea by Blizzard. No skin in the game here as a normal raider, but I've long advocated some sort of difficulty level between LFR, which is faceroll and impersonal, and Normal modes, which honestly aren't for everyone. I'm just a bit leery because the last time they implemented an additional raid difficulty, we got the worst raid in WoW's history (8 bosses, almost no new models or locations).
Things I would like to see happen:
1) The gear needs to be low ilvl enough to be unattractive to normal and Heroic raiders. It's bad enough that I feel I have to run bosses in LFR to chase drops (still have a damn 489 shield), but having a third difficulty to buy wow accounts run would lead to massive burnout among my raiders. I'm already worried that I'm going to have to run an additional flex day to practice on bosses.
2) Scale down smaller in size, 8 or even 5 people for those friends and family guilds. 5 people would probably sound the death knell for 5-mans though.
3) Make it easier to find groups in-game. People shouldn't have to rely on cheap wow gold a website or addons to find groups.
4) Come up with a better name than flex. It just doesn't seem to roll off the tongue.
I actually think it would make some sense to have LFR and Flex share the same lockout. Reality is unless everything shares a lockout (which seems to be pretty torn among the playerbase, some people want it to sell wow accounts, some people don't) people are always going to try and gear as fast as they can.
At a minimum, though, if you locked LFR and Flex together it would at least make it so people would have to stay away from LFR if they wanted the better loot and just by nature of it not being a PUG (or if it is, it's a PUG you have control over) Flex runs should move at a faster pace than LFR in most cases (assuming they have the same amount of trash).