say, for example, cut the rope 2, which was not free but where you had to use consumable powerups to get certain items in the levels (the "clovers") in order to unlock some levels, only after a major outcry the developer changed it so you could get access to the extra levels if you got 3 stars on all the others. You also get a 'daily gift' (usually a powerup or two) just so you are semi-forced to check in every day, and there are also other obnoxious mechanics so as soon as you spend a little bit of time thinking about a level the "level solution" powerup starts blinking annoyingly. And this is on a non-free game!
I had insta-bought all previous cut the rope games pretty much and 3 starred most of the levels in all of them (great mechanics, om nom is cute) and I had gotten a lot of my friends into them, but I have honestly given up in disgust with cut the rope 2 (only gone through 1-2 worlds) and will not give the developer a dime for "powerups" or any further games they will release.
In terms of "pure" f2p I am actually enjoying hearthstone, I had never played a card game before but it is definitely fun (after you lose enough games to get matched up with similar "f2p" opponents without tons of rares/legendaries), it took me losing about 15-20 games in a row before I ended up at a level where I more or less win 50-60% of the time and my opponents also only have "standard" cards. I figure blizzard is losing money on me as a player, but I figure the wow subscription I have been paying for many years more than covers this (if at all I think Blizzard should give a free card pack every month to WoW subscribers as a random gift, if it was retroactive it'd be even better