First off, a statement: One of the major complaints to non-raiders prior to tBC was that forty people was too many for most guilds to field. There is a limited pool of players who want to raid per server. Getting fifty raiders (you need five to ten more than a dungeon calls for in your guild so if some people are not available in a given night, you're not fucked) is no small task, and in general there are those who really want to be in hardcore or even one of their server's Bleeding Edge raid guilds which lowers the available pool of potential members even smaller.
Following theory: With tBC reducing the raid sizes from twenty and forty to ten and twenty-five, it would make raids more accessible to the causal player base. Side note: This led to much bitching by the elitist player base.
Reality: While lowering the maximum number of players did make raiding a more viable option to smaller guilds, it had another effect (whether the devs realized this or not). That effect was that there was suddenly much less slack.
Let's say you have a forty man raid. Not counting the (on average) twelve healers and three tanks, you have twenty-five DPSers. Let's say the boss has 1,000,000 health, so each DPSer needs to do approximately 40,000 damage. If one of those players disconnects, or is undergeared, and is only able to do 20,000 damage by the time everyone else does 40,000 damage. The other twenty-four players need only do a little over 800 damage a person to pick up for the slack by this one person.
With less people overall in a raid, there are less people to pick up the slack, so if one person disconnects or is very undergeared everyone else needs to pick up a lot more slack. It's one thing when it's five or so people out of forty. But five people out of twenty-five, especially when ten or so of them are healers? That's a much larger percent. As a result while it's possible for smaller guilds to consider raiding, each raid is now a step up in terms of difficulty.
Then there's Karazhan. Karazhan is, as we know, a ten man raid. The normal composition is usually a main tank, an off tank, three healers, and five DPS. There are some room for leeway in those numbers, like having a paladin or a bear in a tank role and having them be willing to heal if needed, or having a DPS warrior in the off tank position, etc. but that's the basic setup. As a result, if one DPS disconnects, the remaining four people need to put out 20% more effort to pick up the slack. One healer disconnects, and it's probably over, unless one of your DPSers can also heal.
Considering that Karazhan was meant to be the intro raid for tBC this did not give players who were not at least somewhat dedicated to raiding a good first impression. It's my opinion that the first raid instance of any expansion should be a somewhat easy (not too easy) twenty-five man raid that will give people a bit of slack to get geared up and up to snuff, without viciously punishing the smallest bit of bad luck.
If you put a ten man raid in the game, add it as a lateral progression in terms of challenge, but with slightly worse rewards. Also, the devs would need to say that this was their design. The reason you say this is because it's most player's nature to think of things linearly. Like so: 5-mans > Zul'Gurub > Molten Core > Ruins of Ahn'Qiraj > Blackwing Lair > Temple of Ahn'Qiraj > Naxxramas. Now, anyone who raided back in the day knows that you don't need to go to ZG before MC. Obviously you don't because there were tons of players who made their way through MC before ZG ever saw the light of day. But after it was released, it was interpreted as a stepping stone between five-mans and MC. So basically the devs need to be brutally clear that it is a lateral option, not part of a linear progression. This would also involve not having any attunement chains involve getting an item from one of the ten-mans.
Also, as another issue of note with Karazhan as the intro raid: It's only a ten man raid. You start off with small group, and then you hit the later raids, it's something of a shock to suddenly have larger groups, plus it's not good training for new raid leaders. While a disconnect of one person is disastrous to a ten-man raid, it's a lot easier to direct ten people. Once the same leader has reached twenty-five people, that's much harder. Having them start out with twenty-five players then going down to ten would nice and calm, instead of a huge shock.
Finally, it's also not just that the problem is that Karazhan is a ten man, but that ten is not evenly divisible by twenty-five. So what you ask? Let's say that you have twenty-five people for a raiding guild. Perfect world and all that jazz and everyone's always there. Five players of that group are denied the right to go raiding a week, due to pure numbers. In our imperfect world you will need thirty to thirty-five raiders for a full raiding guild, but of those players you can still only do two runs. If you did three runs, you wouldn't be sure you'd be able to replace the right number of people, because when someone else steps into a Karazhan raid they are locked to that raid. Instead, I think the best numbers they could work with would be the "full" raid would be thirty people and the "small" raid would be fifteen.
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