AVEE'S PROFILE

Having a good amount of options right from the beginning is enough for me to get hooked, at least until the story and atmosphere are set (if these suck, I'll stop playing no matter what), like starting with 3-4 characters, each having a few skills and special abilities, already available customization, etc.
Otherwise if the pace is slow I'll hate the game.

@LockeZ: Agreed.
Now what about a game that offers a constant peak of excitement, or at least a curve that is constantly rising? Provided that the difficulty is well-balanced, that would surely make for an addictive game.
The best example I can think of is Tetris. Someone who played once and enjoyed it probably has played several dozens times afterwards.
Would there be a way to make that work with an RPG?

I like the idea of enjoyment curves, with peaks of intense fun and valleys of less intense moments. Designers should try their best to make every single moment of their game enjoyable, with intensity of enjoyment being in a wave pattern:

During intense moments, players focus a bigger part of their mind into the game to make sure they experience it as fully as possible. These peaks typically correspond to battles or plot twists. These high intensity moments will make the stronger impression and be what the player will most likely remember of the game. If these are enjoyable, good mouth-to-ear advertising will follow. If these moments last for too long, players will get mentally exhausted, find the game too difficult or have a hard time keeping up the pace.

Whilst in the quieter moments, players have the opportunity to "digest" the previous more intense moment, mentally prepare for the next one, and take a step back to think about strategies, the storyline and other aspects that were momentarily put aside in the player's mind during the more intense moments. These valleys typically correspond to exploration, planification or dialog scenes. If these moments last for too long, players might become bored.

I think the key is to balance these curves so that players will neither get exhausted nor bored.

Regarding gratification, the players should seek it in the "journey" and not in the "end". Whether the gratification is instant or delayed the road has to be enjoyable, as LockeZ pointed out...

If gratification is delayed during a high intensity moment, players will see this moment as a skill-testing challenge to overcome in order to receive a reward. Here, designers should have the players focus on strategy and overcome difficult situations rather than spend large amounts of time repeating a sequence of actions.

If gratification is delayed during a low intensity moment, players will see this moment as a more relaxing challenge that will most likely require planification, puzzle solving and patience in order to get the reward. Here, designers should have the players test their knowledge of the game (observation of the surroundings, memory of hints in dialogs, etc.) and allow them to find new things (equipment, abilities, etc.) to diversify the gameplay experience.

The rewards themselves must not contribute to the boredom either.
For instance, getting an overpowered weapon that breaks all strategy for the remaining of the game is a no-no. It will only make the game unchallenging and boring, unless you don't use or sell the weapon, which would make the challenge near useless afterall.
Grinding and raising abilities so you can beat the next boss to a pulp is a player's deliberate choice of making a potential "peak" a "valley". Thankfully, players usually have the choice here, so everyone can enjoy such a moment the way they want.

As an interesting example, think about the older NES's and newer systems' Ninja Gaiden games. In the older titles, every second of gameplay was a "peak", moving forward in the stage avoiding and beating enemies. These stages were relatively short and very challenging. The "valleys" were the cutscenes between stages. These moments could seem long but where so good looking that no player ever got bored during these cutscenes. To me, the NES Ninja Gaiden games are a great example of perfectly balanced enjoyment curves.

On the newer Ninja Gaiden games, to the fighting moments and cutscenes were added exploration moments, where you wandered through the stage in search of the next path and could find items. These are "valleys" and should be short and challenging: getting the items should require good observation of the surroundings and some skill with the controller.

Oh I know!
Mr. Pepper looks like a drug dealer so... he can make Kazoo sniff some and with the boost he'd fly me all the way back to the civilized world ^^

kentona seems to crave for a game with character classes, huh.

Well, I can think of one reason why some players would want to start over: to challenge themselves and try to get a better completion rate faster. Using Metroid again as an example, the higher the rate and the lower completion time are, you get a better (or should I say sexier) picture of Samus at the end.
Of course a picture is a weak reason for starting over the whole game, but it could be interesting if the reward was a completely different and more satisfying ending.

But still, that would only appeal to the kind of players who care about testing their skills at a given game.

As supremewarrior said, unlockables can be interesting too. Provided that the game is relatively short and the unlockable content is worth it, it would be reason enough to start over to experience new scenes, plots, characters, quests, abilities and all you couldn't do during your first run. Designers should then make the game's world and storyline more intricate and divided into branches, I guess.
Can't think of a good example at the time... maybe Valkyrie Profile? Nah, there's no unlockables per se but there are 2 distinctive paths and endings.