Tangible Things

Try to represent the game state and information with pieces, which are either in play or not in play, or at a particular place.

Avoid representing the game with abstractions, like text and numbers. Try to use very little text, and try to use less numbers. (Numbers are vastly better than text, as they're very information-dense.)

The components are the game.

It's much easier, and far more entertaining, for players to see physical objects, than to comprehend and parse information.

Imagine a card that has text that says "Unique Creature — Minion Fire Shadow Berserker" and has five different attribute scores. It has the ability "Other Shadow creatures you control get +1/+3". I'd rather the same complexity be spread out over five simple cards. Instead, I have a Wizard card with three spell tokens on him, I have an elf with strength 3 and a sword card, and I have a horse token.

Types

In games centred around cards, you usually need a few card types (like "Creature" or "Item".) These types behave differently.

Almost always avoid subtypes, factions, or other arbitrary categories with no inherent behaviour. These are not tangible, and often arbitrary. Also, avoid making things that care about such categories.

Radlands does have water costs, which are numbers. However, in Radlands, there is no "health" number. People are healthy (upright) or damaged (sideways.) When they die, they are discarded. They're weak, but you have a lot of them.

There are no "factions". Cards just work on people or camps, or they refer to tangible things. Exterminator destroys all the damaged (sideways) people. The default damage ability can't hurt people who are behind other people (being physically behind something is tangible.)

All cards have an icon in their corner. This is what you get if you throw the card away. It has no meaning once the card is in play. I could've used these as different "factions", like "destroy a person that has the raid icon". The icons are already there, so I wouldn't need to add anything. However, they're not tangible. They're information. So, I never used them that way.

If your game lacks depth, just add new objects, not intricate information.

Use very simple pieces, but more of them.

Theme and psychology

Tangible objects are also much more thematic. Each can be given art, design, location, size, and shape.

People also want to manage physical objects.

In my farm game, the trees grow, by adding tokens. The bandits spread out from their ominous fort. The fish eggs flip over into fish. Most of the game is tangible. The playtesters love this tactile engagement with the game.

You'll need some systems that are intangible, but minimise them. 

In my adventure game, combat is thematically necessary. But there's no combat system. You just draw a card, and if it's a bandit, you must pay him one coin, or lose five health. That's it. You might think that sounds extremely simple, and not complex enough to make an interesting game. That's correct. The complexity is elsewhere, not in a combat system with "block" stats, bonuses, and convoluted mathematics.