How Complete Is It? It also keeps track of various percentages of completion: different monsters defeated, number of recipes Alchemized, amount of wearable items you've owned. It invites you to be a completist, and then tells you how well you're doing at it. This is a small, fairly common game mechanic, but it's still motivating. Ironically, DQIX also offers achievements, but these semi-external motivators aren't interesting to collect on their own -- they follow naturally from the intrinsic motivations that the game has already provided, such as its class system.
A Robust Class System. It's been 20 years since Wizardry VI, and it finally has a worthy competitor for the crown of best and most interesting class-changing system. DQIX uses a similar mechanic to Wizardry VI/VII in that skills you learn in one class transfer over to the next when you change (it's not as complex, however, in that you really can't mess up).
It's to your advantage to change classes regularly, as this piles up skill points as well as opening up class and skill-specific quests. So it's to your benefit to to switch to classes your characters haven't yet used, and therefore to go back to different areas to level them up. Sounds boring? It probably would be, if there weren't also benefits to going back to already-explored areas.
Non-Random Item Collection. Certain spots scattered across the world map have specific items to gather. Here's a graveyard with Thunderballs, there's a waterfall with Fresh Water. These items regenerate based on playtime, so every couple hours, it's worthwhile to run around and collect them all again.
The number of each item may change, but the fact that there's some there -- and that this is by far the easiest way to collect them -- doesn't change. If you want to grind this out, there's more efficient paths to get all the stuff, and gain your levels on the way. The only problem is that sometimes, they don't respawn quickly enough.
Controlled Random Item Collection. Most of the items you need are held by enemies, but their drop rate in combat is very low. By targeting specific enemies with a party of skilled thieves, you can ensure that with time, you'll be able to get the items you need.
Thievery has a low chance of success each individual time you use the "Half-Inch" skill, but eventually, you will get what you need. It may take thirty seconds, it may take five minutes, but it will happen. In older RPGs, this could be incredibly frustrating. You didn't know when combat was going to occur, or what kind of enemy you'd be fighting.
Enemies on the Map. Dragon Quest IX puts the monsters that you fight on the map, making it easy for you to target the appropriate enemies. Goodybags from which to steal Brighten Rocks occur maybe 1-in-10 times on the map where they show up initially, which would be ghastly in random combat, but not here.
Want to find metal slimes to grind experience? You can run by all the other enemies to fight them. Of course, the slimes themselves are tougher to kill than most enemies.
Uncontrollable Randomness. Metal slimes (and other similar creatures, like Metal Medleys) offer much more experience than other creatures that can be found at that level.
However, they tend to flee combat randomly, making hunting them much less controllable than any other monster (You can do a few things like equipping random instant-kill items to increase your odds, but they're still low). The experience haul is still worthwhile, though it of course can be frustrating.
Likewise, certain blue-colored chests scattered throughout the world offer almost totally random items when opened. They respawn, much like the spots on the world map that offer Alchemy ingredients. Getting lucky with a blue chest by picking up a higher-level Alchemy creation, such as an Enchanted Stone, can save you 10 or more ingredients.
By offering different levels of chance and control to get the Alchemy items, Dragon Quest IX manages to provide equally varied methods of motivation. Each one applies its form behaviorist psychology to your motivation.
Hunting metal slimes, repeatedly stealing the same items, and exploring the same area of the map and opening the same blue chests every couple of hours each have a finite amount of appeal on their own. But together, they give you the ability to continue grinding in a slightly different fashion. Each of them help to build your characters and item collections, so cycling between them becomes natural.