Usually, unlocks and rewards are put into games to get people to grind for more points. Pople end up playing more and more even if they aren't really enjoying the game all that much, the achievements and unlocks drive them in some way.

I'm in the CS:GO beta, allow me to share my experience with you. The first day I loaded up the game, I failed at it really hard, I couldn't kill a thing and always ended up at the bottom of the leaderboard. I played and I played but nothing seemed to work out. Now if it was one of your run of the mill shooters, I would've been unlocking cool gadgets and unlocks and I would've slowly gotten stronger if those weapons give me an advantage in some way. Here my xp metric in-game is helping me out. But that doesn't happen in CS:GO at all, this is what you have to decide. Would you like your game to take the players' hands and lead them forward or let them 'learn' and gain 'experience' in the real world sense, and become better at the game? The former is considered safer in today's times, this is what makes older games a lot more fun for some people.

After spending a couple of more hours in the beta, I finally understood the ropes, I got a feel of the recoil on every gun, I learned that my accuracy is pathetic while moving, and crouching while shooting helps a lot. It gets better if you pay more attention, some guns have a directional bullet scatter, sort of like a biased random deviation on them. Its very satisfying to figure these things out, but not for most gamers. In a survival shooter, all these things can be applied quite easily. You don't have to spoon feed all the facts, for example, dismembering limbs slows movement, headshots are fatal (duh), aiming for the eyes makes them blind, and the special ones with the fatter arms explode to deal damage to their allies. Stuff like that, I'm sure you get the drift.

there must be a progression and a little balance shift, because after I play for weeks, I want to feel stronger.

the game that balances while I play, I mean "exactly balances" anything I do, is not something that attracts me. It's not power playing, just rewarding.

the progression must be made in a way that it doesn't ruin the game, that's why it's probably the most difficult part of the game testing.