Also, how many different items should one monster be able to drop? Additionally, if that number is high, I suppose I should throw in some useless, sell for money, loot.
I think 5 is pretty good. So common, uncommon, rare, very rare - and super extra ultra rare, for that special kind of item you want the player to grind hours for.
I think it's also important to consider whether or not your target audience wants to grind for hours. Generally, I don't like the idea of making it possible for the player to put loads of effort into the game and get rewarded with nothing, so I don't really dig the whole "very rare random drop" thing.
For instance, if you have a synthesis system, you'll want a category for ingredients. Or if you have some kind of hunting quest system, you'll want a category for "proof of kill".
Currently, I have items which are used/mixed. Then, a few components for synthesis (not many, it's only for weapon upgrades currently). Then, I could add the monster loot that you sell ... although that always felt useless and pointless to me.
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