I have concluded what (In my opinion) constitutes perhaps the biggest difference between Vanilla/BC and LK/Cata when it comes to the 'epicness' of the first and the 'non-epicness' of the latter.
When I started playing in Classic, and I think when most players start playing, they understand the concept of raiding as a fight between players and baddies. There is ONE set of baddies that you must kill each time. When you do so, you win/you gain loot/you feel satisfied/you have achieved something/etc. This was the model of Vanilla/BC.
The model changed however with LK/Cata. Now, there are MORE THAN ONE sets of the same baddies. In some cases, there are as many as 4 different sets of baddies: e.g. 25-man ToC, 10-man Toc, 25-man ToGC, 10-man ToGC. This reduces the epicness - no longer can you feel satisfied/achieved/etc by killing one set. No longer can you say (to others, as well as to your self) 'I have killed C'Thun'.
Now, this new model in LK retained some of the features of the previous system. "The boss" was predominantly the 25-man version of the boss. It was harder, it dropped better loot, etc. You could still, to a certain extent, say "I have killed Malygos", if you did so in 25-man.
With Cata, things got worse. The old model was scrapped altogether. Now, 10mans and 25mans are 'equal'. There is no longer the idea of 'killing the boss'. Yes, Paragon killed Ragnaros HC 25man, but MOST of the people have ALSO killed Ragnaros. What? It's not 25man hc? So what? They have still killed the SAME guy, with the SAME name/skin and many of the same abilities, in the same raid instance. In older times, the majority of the player base would not EVEN SEE a particular boss - not to mention, kill him.
Long Story Short: So, my view is that in order to regain the epicness of the game, there must only be one version of each boss. As in real life, where there is only one unique version of every thing. No 25/10 mans, no normal/heroics. There can still be some hard-mode achievements to discriminate between the exceptionally good and the good. But the basic aim of the game should be 'to kill the boss'.