Some people have already answered this, but it can never be repeated enough since there are always those out there spreading misinformation and pooping all over the party, but the Dream is not a city-less reflection of modern day Azeroth. The Dream reflects the original version of Azeroth that the Titans envisioned - it is a spiritual plane that takes the form of the original single mega-continent that existed before the Sundering broke it up into the separate lands we explore currently.
People often complain that the Dream would be 'just forests/Azeroth,' as if that would be a bad thing - most of the zones in the game could be described as some kind of forest. Pandaria's climate was more of what we'd seen already, and it was still magnificent. But the Dream
isn't just a big forest - it contains every kind of climate imaginable. Forests, mountains, jungles, deserts, arctic tundra, seas, etc. The Dream's zones wouldn't be 'all green' any more than Northrend's largely wintery landscapes would be 'all white'. And it could still be cool even if it were - imagine a desert with green sand and black obsidian mountains. Be pretty cool.
It is also a highly magical place outside the laws of material Azeroth, so we can get weirdness like this:
Think Outland level weirdness and then some. Huge opportunity for creative environments completely unlike anything we've ever seen before.
As for a compelling reason to go there - the Nightmare still exists. Cataclysm made it clear in multiple questlines that the Nightmare is still in the Dream and has begun to spread through it once again. It is also mentioned in fiction outside the game that the Nightmare is searching for another entity to become the Nightmare Lord and direct its forces. Ysera's depowering only opens the way for the Nightmare to spread further, and for us mortals to get in on the action.
Frankly, I'd rather see the Emerald Dream than the Legion, or even Azshara.