Synergy

Synergy is when the end result of multiple efforts applied together is greater than the sum of those efforts individually. For example, if you have a few Valium or a few drinks, it'll help you relax, but if you take them together, you won't know what's what until you wake up dead. That's synergy.

To use a less grim example, cooking. Certain combinations of foods will taste better together than they will individually (i.e., B+L+T). Likewise, certain combinations of otherwise delicious food may taste wretched together (use your imagination). In part, this is what makes cooking interesting, that you can't just roll up all the best ingredients into a ball of 'win' and marinade it in awesome sauce, but you have to be mindful of how each ingredient interacts with the others. This also makes cooking a decent meal more complicated than equipping your party in nearly all RPG's.

Consider Final Fantasy IX. The game features an interesting system of passive abilities. Each character in your party can learn dozens of passive abilities, and can have several of them be in effect simultaneously at any given time. With that many choices, there is a staggering number of different combinations of abilities for even one character. Unfortunately, none of the abilities interact with each other synergistically, so choosing a set of passive abilities is often pretty simple; you're free to just choose your best abilities without worrying about how they interact with each other.

To be fair, there is one notable exception to this. "Auto potion" allows a character to use a potion (or high potion) out of turn after each time they are damaged, but even high potions heal too little health by midway through the game for this ability to be worthwhile. "Chemist" doubles the efficacy of certain items used by a character, but it still doesn't make those items good enough to warrant spending a turn to use them. Individually, these abilities aren't the greatest, but they complement each other such that they're viable when used together.

A game that uses a similar system much more effectively is Paper Mario: The Thousand Year Door. The player can equip Mario with both combat techniques and passive abilities, though in Thousand Year Door this done by equipping badges because Nintendo refuses to just call things what they are. Rather, they would have you believe that Mario simply wears his boyscout sash everywhere so he can constantly rearrange dozens of badges without poking too many holes in his overalls.

But the point is, many of these badges interact with each other in exciting ways. Finding one new badge in that games doesn't necessarily just give you access to one more effect, but may give you access to multiple new strategies, depending on what other badges you can combine it with. Some sets of badges may significantly change Mario's combat role, completely changing the basic battle tactics the player uses, or sometimes a group of 2 or 3 badges can make a badge combo where they have a powerful effect together that is qualitatively different from what they would do alone. In some cases, badges have subtle anti-synergy. A badge that helps you dodge attacks doesn't really play well with one that increases your defense, since every attack you dodge was an attack where your defense wasn't relevant.

The most notable badge synergy in Thousand Year Door is the 'Danger Mario' build. The game has many 'danger' badges, which have powerful effects that only trigger when Mario is almost dead. Individually, these badges are trash, since being 'almost dead' rarely lasts very long, as you'll generally either heal yourself or just die within the next turn. However, if Mario has all of the 'danger' badges equipped at the same time, then once you become almost dead, you become nearly unstoppable. The most important of the danger badges is 'Power Rush,' which increases your attack power by 2 (which is a huge boost in this game) when you're almost dead. The unfortunate thing is that with a little patience, you can buy as many Power Rush badges as you want, turning Mario into the Glassiest Cannon that ever there was and making the rest of the game comically easy. Like, kill-the-final-boss-in-one-turn easy.

So maybe Thousand Year Door takes synergy a little too far. But still. Points for trying.

Inventory

Inventory in RPG's is handled in many different ways. Some of the more traditional methods include 'infinite of anything,' as in most Final Fantasy games, where there is practically no limit on how much your party can hold. Similarly, some games, such as the Tales of Whatever games, use a 'little bit of everything' system, in which the party can hold any number of different items, but there is a limit to how many of the same item you can have. The last one I want to mention, and my personal favorite, is the system used in a handful of RPG's, such as the Mother games, in which you have a limit on the total number of items you can carry, and none of them stack. For example, lets say you're playing Mother 3, which has an inventory limit of 30 (I believe) for a party of one. Say you were traveling by yourself and harvested 30 pieces of dung, you would not have space in your inventory for even one loaf of bread, as may be the case.

In my opinion, the most important criterion on which one should judge an RPG is the rate at which the game forces the player to make difficult decisions. A game using an 'infinite of anything' inventory system doesn't really force the player to make any interesting decisions while she's standing behind the counter in the item store; the answer to the question "what do I buy here?" is almost always "more than enough of everything." Consumable items are usually inexpensive, and money is easy to come by in games where you can just grind for a few minutes outside of town till you have what you need (which is most RPG's). Games using a 'little bit of everything' model are little better. In this case, the answer to "what do I buy here?" is always "as much as the game will let me." Item shop decisions get much more interesting in games that impose harsh limits on your inventory.

Consider the game Evolution, an RPG for the Dreamcast. Your inventory space is severely limited, and to make things even trickier, money in the game is semi-finite. When you browse the item shop in Evolution, you have to ask yourself a lot of questions about the dungeon you're about to go into.

What status effects will I have to deal with here, and how frequently will I be inflicted with them? If one of my characters does get a negative status effect that I'm prepared to cure, is fighting with that status affect worse than spending a turn to cure it? Am I more concerned about healing light damage in multiple characters, or heavy damage in one character at a time? This depends on what monsters you are going to be fighting, as well as how you've chosen to set up your party. The answer to all of these questions will affect which items you buy, and in what proportions. Instead of being an errand you have to run before diving into the dungeon, the item shop is a meaningful part of the game.

But what about items that you don't buy in stores, those super rare consumable items that you save for a rainy day because they're just too awesome to use? Evolution has those too, but they don't usually invite you to make interesting decisions in the same way. One such item is the green moss, an item that you'd be lucky to find five of over the course of the game. Being that rare, most players would be compelled to save them for a potentially difficult battle in the unspecified future. It works out well enough in this case, since the only place where they're particularly necessary is against the final boss of the game. If you have around four or five of them, you can use a tactic that ends the fight in a few rounds when it would otherwise be a difficult battle. Until then, the player can just keep them in the storage locker in the protagonist's house, as not to waste precious inventory space on an item she probably won't be using any time soon. I imagine most players appreciate the storage locker, but as I see it, the storage locker robs the player of a chance to make an interesting decision.

Consider Super Mario RPG, Legend of the Seven Stars, the quirky offspring of Nintendo and Square. Many consider it a Beginner's RPG designed to initiate eleven year olds, but it has one of the most unforgiving inventory systems I've ever seen in a game. Your inventory can hold a grand total of 29 items, nothing stacks, and you have no storage. This makes things complicated right from the start. Ten minutes into the game, you are practically handed a Kerokerocola, a rare, powerful healing item which is also too awesome to use. It fully heals all members of your party, but at this point, your party consists of only Mario, so there obviously must be a better time to use it. Problem is, since there is no storage, as long as you are holding on to that cola, saving it for who knows what, you have effectively reduced your inventory to 28. It only gets worse from there, since this game is full of super unique items: a membership card to a top secret casino, a magical money bag that has infinite coins in it, etc. They all take up space in your already tiny inventory. For each of these rare or unique items in your inventory, you have to choose between long term security, knowing that you have a very useful item in case the situation should demand it, and the immediate benefit of extra inventory space that you get from discarding the item, never to be seen again.

Unfortunately, the game just generally isn't difficult enough that you'd ever need the extra inventory space. But still. Points for trying.