I should preface this by saying I've only spent about 2 hours in the zone all told, and that druids are fairly broken in the most recent two beta versions, so my experience should be taken with a grain of salt.
That said, I was able to proceed without difficulty most of the time. Unlike in much of the old world, I did not encounter instances of randomly getting shifted out of flying form and falling to my death. Which is too bad, really, as it added a bit of excitement and adventure to my explorations in the Eastern Kingdoms...
The graphics in Hyjal are amazing. I can literally hear my graphics card chug into high gear (well, I can hear the fan, at any rate) when I fly in under Nordrassil. This beauty may be difficult to appreciate with lower spec hardware. The sky is also a work of art (literally) and probably my favorite of any zone so far.
The epic quality of the zone is bolstered primarily by the fact that many of the quest givers and other NPC characters are Big Important People from vanilla who we only got to see back then, if at all, in raids or in the Shifting Sands quest chain. This includes the villains. It's pretty fun to run into old raid bosses wreaking havoc out in the world. There is also a battle going on between the Cenarion Circle (or associated forces....they are a new faction) and the twilight cultists/fire elementals. It's not quite as intense as the opening siege for those who first stepped into Northrend as hordies in the Tundra, but it's not bad. It's also a bit more dynamic than the standard "there are some bad guys and some good guards fighting." There are some bigger characters and druidic types that react to some of the Big Bad Lieutenant abilities, and if you don't follow their lead, it's pretty easy to find yourself extra crispy.
The epic feeling is brought down a bit by the lack of content (at least, I hope it will be filled in before release) in some of the twilight camps. Some of them have no cultists at all....just a few empty wagons. Which is weird, especially when your quest is to destroy the camp.
Some of the initial quest lines also remind me of one of my favorite zones in Northrend - Zul'Drak. Like the quests sending you to each temple to discover the current status of its god and then handle the fall out appropriately (which was great because it gave you an overarching plot line with a bit of tension and excitement, but still mixed it up and had you do something different for each god), you are sent to the shrines of four different ancients (actually, Ancients with a 'A'...not just the big trees, these guys are the remnants of the great spirits of nature that fought and fell in the War of the Ancients) to try and help bring their powers forth to drive the cultists back from Nordrassil.
So far, the zone is beautiful and has a nice epic feel, and most of the quests are even completable (added bonus!). We'll see how it goes from here....it's a big zone.