YHA hostels, otherwise known as HI in the rest of the world, can be pretty weak. That's not the case in Australia and New Zealand, where YHA is known for their quality amenities, although that's sometimes balanced out by an institutional feeling.
The YHA in Cairns is far from institutional. Their Esplanade location is more party central if that's what you're after, but the Central is plenty fun, simply based on all the great people staying there and the friendly staff working there. There are some new mega-resort "hostels" in Cairns that have just about everything including a shopping center inside, but I prefer the homey feel of this place.
Rooms are clean with pretty decent beds - the whole place is a converted motel, so it's got more of a hotel feel than a hostel, but the friendly vibe makes up for that quickly. The kitchen is huuuge, which makes it easy for many people to be cooking dinner at once. Travel services are practically infinite, although you'll still want to shop around with different agents if you're making Cairns your starting point for an Oz trip, as many people do.
The main gathering point is the swimming pool (plus spa and saltwater spa!) as well as the loggia around it. Here you can meet anyone just about any time of the day, which is great, especially for solo travelers. I'd just sit around and drink a beer and next thing you know, have a few new friends to go out with for the night. (or longer
Recommendation: The Central YHA isn't the party hostel in Cairns, but believe me, it's a great idea to have a place to get some sleep, because all the adventure activities in town have an early start. You won't be short on people to party with, and you'll be within walking distance of just about every hotspot in town, so there's no reason to stay at one of the other places with a party rep - unless sleep just isn't your thing.