I will now describe the most accessible raid in the world.
Get 40 people, tanks, healers, and DPS, and go to Accessible Raid, a place in which the bosses do not hit hard and follow normal aggro rules with no phases, adds, or strange debuffs to worry about. The gear requirements are: the starting zone gear, so if you are wearing a single green, you are completely overgeared. There is no need for consumables. No key. No attunement. No strats to learn. Also there is a teleporter from every major city to the raid portal, which is next to a summoning stone in a sanctuary zone.
Now everyone can do this raid.
Oh. 40 people is too many? How about 10? You can't get that many healers? Fine, it's easy enough, just go solo it. But then it takes too long and you don't have time?
Okay, new Most Accessible Raid in the World: It is balanced for one player to faceroll it and takes five minutes, along with having all the previous features. And it can be accesses by a Solo Looking for Raid panel which will teleport you, just in case your computer cannot handle the lag in Darnassus.
Are you having fun yet?
Was that raid accessible or merely trivially easy? Where's the line, anyway?
Molten Core was fairly easy, but FR fights and occasional forced raid makeups meant that it wasn't accessible.
The original Naxxramas was hard and inaccessible with strange tank requirements and consumables so out of control that eventually Blizzard nerfed alchemy, seemingly just in response to Naxx.
Icecrown Citadel is harder than Molten Core and certainly more accessible. Well, that is unless you count the dozens of heroics to run to gear up for it. Suddenly it looks just as grindy as any MC FR gear run.
But what is accessibility? I'm still using this word that I haven't defined. To me, accessibility is the ability to walk into a raid and kill a boss. It's not a number and even ordinal ranking may be impossible, since what makes a raid inaccessible may vary between players. So maybe it's better to ask what makes a raid inaccessible. What makes it harder for players to walk into a raid and kill a boss?
Number of players: as this goes up, so the time gets harder to manage as more and more schedules must overlap just right.
Specific player requirements such as a class, spec, or even just role, as anyone who called a raid due to lack of healers can testify to.
Gear, whether it's FR or EH.
Strategies which must be known ahead of time. By this I mean needing encounter-specific responses which must be done faster than a raid could be expected to figure them out on the fly or reasonably anticipate, such as needing a tank being something we should expect.
Reduce these and the raid is more accessible. But it will lose some fun. A very short raid will let people in, but may end up feeling insubstantial. Needing fewer players will do the same, but also constricts raid design and may hurt the enjoyment players get from working in large groups. Avoiding the need for specific classes also restricts raid design as well as risking homogenization of classes to fit raids. Reducing gear requirements may cause overgearing to happen too easily, trivializing fights and reducing the fun, while also causing players to burn through content even faster and quit that much sooner. Having strategies which can be figured out on the fly, meaning with sufficient buffers of time and health, may force encounters to be too easy, especially if players pull a second time knowing exactly what will happen.
Accessibility reduces fun, but it's also what allows us to get to that fun. Clearly this calls for quantification of both terms and the graphing of graphs with pretty lines to show optimum fun-accessibility points for criteria such as highest average fun over the population, highest fun over raiding population, and whatever will make the game designers richer, faster.