Why are lots of people only moaning about the difficulty? It's more then just another difficulty level between LFR and normal raiding.
1. It's flexible, you are not bound to the 10 or 25 format, but also everything in between which can be great if you have too many for 10 mans, but not enough for 25's.
2. You are required to form a group yourself, and not just assembled from all the people that are in a queue. This will help tremendously with toxic behavior. If someone is an ass (AFK, terrible behavior etc) he will either get a warning from the raidleader, or get the boot immediately. Most likely that person will also get ignored by the others that are in the group making it harder for him / her to get a group in the future. This will mean a good reputation can become an item again, like it was before LFR and LFD.
I haven't played on the PTR and have no clue how it actually is to play in a flex raid, but I'll definitely try it in 5.4, at least with alts.