As predicted, flexible raiding kills normal raiding and takes its stuff.
In WoD, there will be four raiding modes, each on separate loot lockout:
1. Raid Finder
More or less the same as now, with a little bit of scaling tech. I'm not really sure what the scaling will really add, given that every role but tank fills pretty fast. I don't really see fights scaling down from 2 tanks to 1.
What I would like to see is for signing up for multiple instances to be the norm, more like Dungeon Finder. For example, let me sign up for all of the Siege of Ogrimmar instances at once. As long as I get put in an instance that still has bosses I haven't killed this week, life should be okay. It would mean that the pools for each wing would be much larger, and raids able to form quicker.
Also, I would like to see four tanks, but that's very unlikely.
2. Normal Raids
What's currently Flexible difficulty. Scales from 10 to 25 people.
The only really interesting thing here is the loot system. Apparently Personal Loot will become an option like Master Loot or Need Before Greed. Otherwise the amount of loot you get will scale. I gather this means that--if you get 1 item per 5 raiders--11 raiders would mean you will have a 20% change of getting 3 items, and an 80% chance of getting 2 items.
3. Heroic Raids
What's currently Normal difficulty, but with flexible scaling. Scales from 10 to 25 people. Pretty much the same as WoD Normal, but more difficult.
4. Mythic Raids
Current Heroic difficulty. Fixed size of 20 players. Having a fixed size should allow Blizzard to make more inventive fights. As well, the race (at least in the West) will have a common format. One nice thing about 20 players is that you should be able to take your bench along when you farm a Heroic Raid.
This does suck a bit for current Heroic-10 groups, but the scaling of the lower level should allow them to work back up to size if they wish to. As for current Heroic-25 groups worried about cutting people, the truth is that there is always churn around expansion time. Some people will want to retire, some will want to come back. The drop from 25 to 20 should be taken care of with normal attrition.
There are also several high end raiders worried that the separate lockouts will mean that they will need to run every instance. I think that the fact that the lower difficulties are flexible will make it easier, it make showing up more optional. You can have smaller subgroups run the lower difficulties, without really affecting things.
Personally, I like separate lockouts because you don't have to worry about time as much. You just finish one instance and then start on the next. When you want more time on a specific instance, you just drop a lower instance from the schedule.
Questions and Conclusions
I wonder what will happen with Thunderforged/Warforged gear. As well, how will Valor be handed out? I wonder if it wouldn't be better to just drop these two ideas altogether. Just go old school and rely on drops entirely.
All in all, I think this is a solid plan for raiding. I think the Flexible raids will make roster and attendance issues much less of a headache for all non-Mythic guilds. Even Mythic guilds should find that the Flexible nature of the Heroic raids will make organizing farming runs a fair bit easier, allowing them to focus on the truly difficulty content.