Things have changed a lot over the years, the way I've come to think of it is "synthetic" vs "organic" engagement. Games (at least modern WoW) are now largely built around synthetic engagement.
Synthetic engagement:
- You come back to do prescribed activities that the game dictates
- Timed quests (dailies)
- Limited events and other incentives to log on often, leveraging fear of missing out
- Unpredictable loot outcomes, and often a disconnect between effort and reward (ie; no way to target certain loot you want, like Titanforging - amazing items can drop anywhere but the chance of any one item is very low)
- Extremely variable difficulty with a baseline of very easy
- Super fast pace, action all the time
- The player is "special"
Organic engagement:
- You come back to enjoy occupying the world and spend time with your friends, or to do specific activities you know might net you specific rewards
- More focus on player interaction
- A mix of solo and group content
- Generally slower pace (longer travel times, longer dungeons, less short-term focus needed)
- Simpler loot systems that rely on bigger, more meaningful upgrades rather than a flurry of smaller ones
- The player is not "special" - they're just another person in the world
- Less of a sharp difficulty curve - more "tough but fair" - keep a few key principles in mind and you'll do fine both in groups and solo
In general, Classic WoW hooks you because it has an immediacy - get better loot and level up and you
notice it. Then you add in a social element - the fact that it's easier to work with others than not (no mob sharing, no group finder, no anonymity via sharding and phasing) and you wind up coming back on and getting hooked because you have an in-game network that opens doors and provides a sort of emergent adventure.