Not long ago, Wargaming.net announced that they were removing "pay to win" elements from World of Tanks. This was almost certainly a smart move; the feeling that a game is pay-to-win is a key driver of rage-quit moments, when players decide to leave because the game is pissing them off; if you're a free player, or one on a budget, it feels hateful if people are beating you just because they have more money than sense.
Clash of Clans handles this in a clever way; as in many games, you can speed your progress through the game by paying to bypass timers, but when it comes time for a battle, you are always matched against a player of roughly equivalent power (based on town center level). If I have a level 6 town center, I will be matched against a player within a couple of levels of mine -- and it doesn't matter whether they got there by grinding or paying, it will still be a fair battle. Paying advances you quicker, but it doesn't give you an advantage when the battle begins.
A friend who played CityVille once told me "I paid $50 -- and it's gone." In other words, he liked the game, and was willing to pay -- but afterwards, regretted it. He didn't feel as though he'd gotten value for money. And soon afterward, he was gone, too; he quit the game.
Clash of Clans is one of the few games to have monetized me. It gates the number of buildings you can have under construction at one time with "builder huts." You start with one, and purchase another with hard currency (a forced purchase) in the first user experience. You see the value at once; more huts means a permanent, secular, continuing speed up in your ability to progress. Additional huts cost hard currency, and you can grind for hard currency by completing achievements; interestingly, the achievement rewards are rich enough that you can build a third and a fourth hut by grinding (though the fourth hut won't come in until the elder game, by which time you are more likely to be resource constrained than held up by too few builders). But for $10, you can buy a third hut immediately and have enough hard currency left over that a handful of days' grind will get you a fourth. That's pretty reasonable, by the standards of F2P. I liked the game, and was willing to do that. (Paying to bypass timers, not so much, but then, I'm no whale.)
I appreciated that I had the choice: That I could grind, if I preferred. It felt like an honest and reasonable transaction. Interestingly, Backyard Monsters: Unleashed, a similar game, makes your third builder a real-money transaction, and doesn't let you grind; I'd be interested in knowing whether this works better or less well. It's still an honest offer, but CoC's willingness to let me grind made me like the game more; Supercell knows how to monetize, but it pulls gently on the levers. (Well, at least until the elder game, when the slow production rate of dark elixir provides another strong temptation to spend.)
Similarly, League of Legends lets players play some champions for free each week, on a rotating basis. This exposes players to different champions and their advantages and disadvantages; players actively like the opportunity to experiment with them. This undoubtedly leads to additional sales, when a champion is no longer free to play; players who enjoyed that champion's play style will be motivated to purchase it. Try before you buy, in other words; but in a way that players find positive and unobjectionable.
Many have decried the level of grind in F2P games, but this isn't a characteristic of the business model; rather, it follows from the fact that virtually all F2P games are "games neverending," like MMOs. That is, they do not come to an end and have no winners or losers. In a game neverending, you need to spin out gameplay as long as possible, because you have limited content, and even in the best case, your players will consume your content more rapidly than you expect. That's why timers lengthen in the mid-to-late game in games that have them; that's why leveling in WoW requires grinding through mob battles.
Grind is not ideal from a game design perspective; but it's pretty much a necessity for games neverending. (Which is a reason why I'd love to experiment with F2P games that do come to periodic conclusions, but that's another story.)
And it's acceptable to allow players to speed up or bypass the grind for purchase; what isn't acceptable is make progress either impossible or insanely difficult without paying. Some levels in Candy Crush, for instance, are (seemingly) purposefully designed to require dozens or even hundreds of attempts to get past unless you pay for additional moves or powerups.
Interestingly, King seems to have nerfed some of these levels over time; I have no doubt they responded to player complaints, but also that they A/B tested the change. So I think we can posit that, yes, pulling on the monetary levers more gently has been successful in increasing their long-term customer value; better to keep them playing with the hope of monetization in future than cause them to rage-quit.
Of course "insanely difficult" is a fuzzy phrase; but the point is that you've made a promise to players that they can play, and grind, without paying, if they so chose; you have to keep that promise, and that means more than just "you have a tiny chance of being able to progress."
Marvel Avengers Alliance has monthly "spec ops" releases. A spec op is a series of linked battles (with a story arc attached); players who complete the series within a set period of time, usually a couple of weeks, get a new playable superhero for free.
However, each of the battles in the spec op requires expenditure of a currency available only for the duration of the spec op (called "unstable ISO"). Each time you initiate a battle in the series, you expend some of it. Players are seeded with a certain amount, but not nearly enough to complete the sequence.
Players can gift each other unstable ISO, but the amount a player can receive per day is strictly limited; and unstable ISO can also drop as a reward during a battle. In short, the developers can constrain the amount of unstable ISO a player can earn by adjusting the "receivable per day" parameter, and the drop rate in battles.
There are intermediate rewards over the course of the spec op; that is, at some point in the mission sequence, a player will be rewarded with a rare weapon, or a small quantity of hard currency, or something else of value. Players of the game actively like spec ops, because they provide new, periodic content, there's a tasty reward at the end of the sequence, and even if they are unable to complete it, they can receive the intermediate rewards.
Needless to say, unstable ISO can also be purchased for real money. And since the superheroes rewarded by a spec op are later put up for hard currency purchase, it is possible that, by purchasing a limited quantity of unstable ISO, you may be able to obtain the superhero for less than the outright purchase cost.
Is this system unethical? I would argue that it depends. If the game's parameters are tuned so that, with optimal play (maxing daily unstable ISO gift reception, losing few to no battles, spending the time to grind), a player can complete the series, then it strikes me as ethical: You're giving players more content, and non-payers can still succeed, though they have to work at it.
If, however, the parameters are tuned so that players literally cannot complete the sequence without paying, then it strikes me as unethical. It's a bait and switch; it's like Lucy pulling the football away from Charlie Brown. You're holding out a reward and giving players the impression that they can get there by grinding, but denying them that opportunity. Players will wise up to this, eventually -- and you will lose some who do.