When I play a traditional "pay" game, I trust that the developer want me to finish the entire game. After all, the team slaved away creating all that content and the player should damn well appreciate their effort.
As a result, when I get stuck at some point in a game (Dead Space 2 being a recent example), I trust that there's a way forward and with a little more effort or practice, or perhaps if I retrace my steps, I'll become unstuck and move along the path toward completion.
Because the developer and I are working with a common goal in mind -- get Eric to the end of the game -- I am confident that at any point in the game I will be capable of completing the challenges set forth (no matter how stingy the Dead Space guys are with ammo).
With freemium games, that trust is obliterated. When I can't progress, I am forced to ask a fun-sucking question -- is the developer trying to extort money from me in order to continue? Am I unable to continue because I just haven't mastered some in-game skill or is this simply the point at which I'm supposed to hand over my tithe?
Candy Crush is an obvious (if extreme) example. When you can't complete a level after the 20th try, the player begins to feel that the developer is expressly barring their passage. And at that point, the game ceases to be fun, at least for the generations of gamers who saved the princess.
Then again, my kids (eight and six) just assume that buying gems is how one progresses in a game. It's not unlike when I was eight and pumping quarters into Gauntlet in order to reach the next level -- Blue Warrior's health dropped whether or not you were taking damage.
Everything old is new again.