I generally like Mount Hyjal. The setup is very different from a normal raid instance, with the trash waves, NPC allies, and bosses that come to you instead of you finding them.
However, one big negative for Mount Hyjal is that there is no purpose to the heroes being there. Unlike the previous Caverns of Time instances, where the Infinite Dragonflight is trying to alter history, pretty much everything seems to happen as it is supposed to. The raid isn't a band of heroes, they're a group of tourists.
I don't demand much from the lore in this game, but there's usually a mission behind our ventures into the various raid instances. Mount Hyjal seems to lack any purpose. It feels like Blizzard thought, "You know what was a cool mission/event in Warcraft III? Mount Hyjal! Let's allow players to revisit it." Playing tourist, rather than doing something heroic.
The Infinite Dragonflight are also superb villains. Mysterious dragons, with that awesome reverb effect for their voices. Epoch Hunter's reveal in Durnholde was an epic moment. Aoenus in the Black Morass made a really good argument for preventing Medivh from opening the Dark Portal. In many ways, the Infinite Dragonflight are the best villains in The Burning Crusade, and it was a real disappointment that they did not appear in Mount Hyjal.
The reason I'm writing about this now is that Blizzard is talking about the new Caverns of Time instance coming in Wrath of the Lich King, the Culling of Stratholme. And the manner in which they are talking about it seems to be emphasising the tourist aspect heavily, "The Culling of Stratholme was a cool mission in WC3. Let's have the players go play tourist with Arthas."
New CoT instances are a great idea. They're fun and often unique. But they need to be along the lines of Durholde and Black Morass, giving the players a heroic purpose, rather than Mount Hyjal, where the raid is just a gaggle of tourists.
Also, bring back the Infinite Dragonflight please.