I've come into Cata quite late, just finished Vashj'ir on my toons an am now working my way through Hyjal, but overall I'm enjoying it so far.

Vashj'ir really forces you to think in 3 dimensions as there is lot more vertical movement, whereas Hyjal is similar to the WoW we all know, i.e ground based questing.

The storyline in Vashj'ir was interesting (i quite liked the visions of the past quests), but a bit slow in parts, and sometimes repetitive (oh look, another cave!). I think a better idea would have been to have the submarine as a base of operations in the zone, similar to the flying ships in Icecrown...

Compared to Hyjal where you are shown Ragnoros straight out of the gate, which kind of lends a sense of urgency to questing in the zone, and it just kind of feels...well, more epic.

The collection quests have been ok for me so far, the only one I really hated was one in Hyjal where you had to click on very small, fast moving animals - 10 for each toon. Pretty sure I came away from that one with a bad case of RSI.

Just my 0.02 worth.

Cheers,
S.