This wouldn't really accomplish what you think it would accomplish. You'd suggest not getting the achievement for a raid until you used the same 10 or 25 people for every encounter? That would have a pretty adverse affect on guilds like mine, where we have 15 or so raiders for a 10-man raid, so we sub people in and out depending on who needs what from each boss, who feels like raiding the most, as well as covering for people who arrive late/leave early, and making sure we have reasonable raid buff coverage. Progression 25-man guilds (like world first guilds) probably have 35+ raiders they can pull from to make up their group.

Also, there is no "world-first" achievement, only server first. World-first is recognized by the community, and the raiding community in general wouldn't recognize this kind of restriction. And even if they did for some reason, the world-first guilds would just do even more extensive PTR testing, and arrange their group for what they needed on the hardest encounter.

Also, raiding is a group effort, and as such, playing the spec the group needs is the proper thing to do. I'm not sure why there is such a stigma attached to this for some people. If you want to play the spec *you* want to play regardless, then there are 5-mans and LFR, and normals after nerfs+gearing, which are so easy that it doesn't matter what you do. If you want to clear normal modes the first couple weeks, and move on to heroics, then you should be interested in what helps the success of the group as a whole, not your own personal preference.

For example, my raiding main is a hunter. We've been fortunate that all 3 specs are pretty close this expansion. However, if you want to be a hunter killing heroic Beth'tilac, then you need to be MM or SV, BM need not apply unless you already have an MM or SV hunter in your raid doing the hunter duties (yes, other classes can handle spiderlings but MM hunters do it best). If you want to kill many heroic modes in T13, you owe it to your raid to be SV. I don't care for BM personally, but if that was required to get a kill then I'd happily respec, reforge, and do my best as BM and be happy with the kill. As a pure dps class, it's my duty to my raid to know all three specs and bring the one that helps the raid the most.

People complain because unlike us hunters there are huge skill gaps in other classes specs. A mage that did well in FL with his 1-button spam suddenly can't handle the more difficult fire rotation and QQ's about how he "doesn't like the fire playstyle" because he's bad at playing it. Similar thing happens with BM hunters, since BM is the easiest spec to play people consider themselves "BM hunters" because they aren't good at the harder specs or just don't want to put the time in to learn.