Naxxramas

[ZugzwangZeitgeist]

Coming from a Shaman, the undisputed third wheel in 90% of all raids, I definitely don't feel useless and/or like a statistic in a 40man situation.  :P  Unless of course it's boring on-farm content (MC coughcough).
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In my experience, if we're exploring new content and someone dies in a 20-man, I get worried. Certainly in 20-person Ahn'Qiraj having a player go down early in a boss-fight can be potentially disastrous. Most of the fights I've seen (Moam and Buru) basically require you to have a full roster of 20 people so that you can deal with them before they get out of control. Rajaxx as well is a fight where if people are dying in the early waves of the fight (or hell, if the NPC's are dying), things are going pretty badly.

Most of the difficult raid instances require everyone to be pretty aware of what's going on or they'll either: A) Get killed, or B) Get the raid killed. Usually a combination of both.

I'm actually a huge fan of the 20-person instances in general. They all are very well designed and a lot of fun - better than most of the 40's in my opinion. There's way more weird stuff to explore and do in Zul'Gurub than Blackwing Lair or Molten Core, and you can always get a really strong, competent group without having to stretch and recruit people just for the sake of filling out the raid. They also tend to make much better use of secondary skills like Sap, Polymorph, Entangling Root, Poisons and so on.

I would love it if Blizzard continues the trend of releasing a 20-man instance for every new 40-man, although if Naxxramas is one big instance and not divided into sub-sections, it looks like Naxxramas will be the first unfortunate exception.