Blizzard can never make everyone happy, but they can give people plenty of things to work toward and show off. In this case, there's a date attached to your achievement for when you killed a particular boss. So you can show the achievement, and to the people that it matters to, you can show them by date that you killed Boss X pre-nerf. Beyond that, there are guild achievements, server firsts, and world firsts, plus of course the gear and tokens. How much more do you want? And just how granular do you want the rewards to be?

[Defeated Ragnaros on Heroic Mode, 10 man, no battle rezzes used, 2 people or fewer dead, in less than 5 minutes, and /mooned Ragnaros at 5% health]

You make a great point. Im in a casual but progressive guild. We raid every week. We raid three days a week and have gotten through all of the raids till firelands.
We wiped continuously on all bosses. After some time, it got to be a bore.
After 584953 wipes, you can close your eyes and do it blindly cause you know it so well already.
The server I xferred to wasn't really a great PVE server as it is a rp server so it suffers from the rp syndrome.

It just got stagnant doing the same bosses every single week.
Kara never seemed to get this boring, but Kara was epic as were all the blackrock mountain raids and dungeons in vanilla. Still are epic solo'ng them now.
I guess it just wasnt one boss after another. You felt like you were truly in a haunted castle or down in the depths of blackrock. Now its just go from one room to the next without much changing.