It has rewards varying from costumes, to pets, to weapons. You can pick and choose which ones you want and in the order you want them. And every so often a big boss and all of his minions show up.
Personally, I would enjoy the other two current adventure zones if they had some of the qualities of the Winter Event. Give us costumes and special weapons themed around their respective zone, and have them separate of the Reputation System. (Though there is nothing stopping you from making Reputation projects that demand those items )
There's a lot that could be done with Nukura as well. Have those little fighter things Tholians have appear in Nukura's molten sky and start hunting down players at regular intervals like the Scorpion Fighter in the Romulan missions. Roving teams of powerful combat ready Tholians that patrol the area.
Just... make Nukura and New Romulus more like the Winter Event. That would make them much more enjoyable.