I decided to break this in to two parts. This first post will look at my first reaction the change and how it will impact raiding. My second post will deal with some of the broader topic brought up in the post like Gating and Hard modes.
The Cataclysm news continues to roll today with this post from Nethaera. If you haven't read it I highly suggest that you go and give it a look. It is to long for me to quote the entire thing.
First Reaction: VERY Negative
There is a lot of stuff in that post, and I'm not sure what exactly I was reacting to, but I hated this change from the start. Through out WotLK I've had fairly good luck with 10man groups. My T7 group was one of the best, and was one of the first to complete the Meta on my server. In Ulduar, it took a while for a group to form that fit my needs, but eventually we put together a very good group that cleared Ulduar, ToGC, and is now working well through ICC. We've had some issues along the way with roster make up, and such, but I consider my 10man group to be one of my WotLK highlights. This change will take it a way from me.
I like being able to get the drake rewards and such a little easier then I would be able to in 25mans, but the best part about 10mans is that they are more laid back. We joked around a little more. There was a little competition between the guilds two 10man groups. It was just fun. Plus I was hoping that I might be able to start of Cataclysm by tanking 10mans. Once again, that is all gone with this change.
Second Reaction: Weary but not fully negative
While I am not all that positive on the post, I am a little more rational on the subject. I still see the negatives, but it also opens up the doors for new positives.
Not having the 10man option means I will have 4 extra hours a week to do what I please with. In particular I feel like I've ignored my alts in WotLK. In TBC when I leveled an alt to 70 I would take him into Kara. I would get some gear and a sense of how the class was played. I haven't had that experience in WotLK. I did get a chance to play my Warlock through Naxx but that was primarily because Naxx was so under tuned. To get out 20 man achievements for the meta, we ran it twice in a single weak with half the raiders being on alts. When Ulduar came out the progression on my warlock almost completely stopped. Most of my guildies were in 10man groups that took up there WoW free time and there weren't many alt runs. I also found it hard to commit to any of the alt runs that did happen because I needed to balance WoW with my real life.
Taking 10 mans out of the equation means I can sleep when I need to sleep, and may give me an opportunity to focus a little more on one of my alts when Cataclysm comes around. That isn't so bad.
This will also help me as a blogger. Believe it or not, I hate my gear lists. I know you guys love them. They bring a lot of traffic to the site, but having multiple ilevels of gear in the same tier is a problem. My gear lists are now freaken huge. Having the same gear drop out of both 10man and 25 man should make those lists much easier to produce and maintain. This doesn't mean I don't have concerns though.
My first big issue with this philosophy is the 10 vs 25 balancing. For this new structure to work well Blizzard is going to have to get this balancing near perfect. Unfortunately Blizzard has not shown a lot of skill at this in the past. I am going to go way back to Sarth 3D as my primary example but I've had similar experiences through out WotLK.
Ask any serious progression raider during T7 what the hardest fight was and you will get the same answer every time: 10man Sarth 3D. It was a tough fight. Much tougher then it's 25man version. Most of the top guilds on my server got the 25man version down first and then the 10man version a month later. We were a 25man guild at this point with near BiS gear from 25mans. What we had to do in the end was to stack the group. We sat Casters the first week for a Melee heavy group, and then did a Caster group the next week. There was no way 10man strict guild could have gotten this.
That was a long time ago, and I do think Blizzard has gotten better over time, but the problems are more structural then that. If they keep the rooms the same size between the two versions then 10 mans are always going to have an easier time avoiding damage. not to mention it will always be easier to avoid one Malleable Goo then it is to avoid 2. Sure you can increase ranges of the boss abilities and such but then it becomes overly difficult on the healers because I doubt they would increase the range of their heals
The problem is that 10man and 25 mans are very different at their core. If they are not able to maintain a good balance between the difficulty then you are going to see guilds treating their raid ID like a talent spec. Raiders will fight which ever version is easier. If you can change the raid size mid instance then raid leaders will kick 15 people from the raid to down an encounter.
Destabilization of Guilds:
I think creating one lock out also has a good change to destabilize all kinds of raiding guilds.
What is the incentive to raid a 25man raid? I know Blizzard says that there will be a greater quantity of gear, but how big of a deal is that? A good portion of our gear already gets sharded every weak. If your guild is already highly geared the extra loot provides little incentive to do 25 man. Not only that I would assume that only one of each item could drop as it is now. Therefore, if everyone wants the awesome trinket that drops off of boss X you have more incentive to do two 10man raids because you have more chance of it dropping and having multiple people get it.
The 10man guilds will suffer as well, due to their limited roster size. In my guild we have about a 30man roster. We assume that a couple of people won't be able to make it each week for what ever reason, and only expect to sit 2 or 3 people each raid. In a 10man you will need an back up tank, healer, and at least one DPS. One person can fill multiple of those roles with dual specs, but in many cases the second spec will have lesser gear and less experience.
In the end, one person being absent from a 10man raid can be devastating, but 25mans can handle it better due to the larger roster size. Ultimately I worry that the incentives are not there to keep people raiding 25 mans, but that a 10man raid size is to small to really support serious raiding.