1. Limited time
Limited levels after which you have to wait. Unless you buy a special upgrade pack with real money and then you can play forever.
2. Rare items
Some items you require within the game are so rare, you cannot help but buy them if you want to progress. The game makers want to earn money from you.
3. Spin slots
There are those ENTIRELY CHANCE-BASED spins or pick a reward scenarios which are so NOT chance-based that you keep getting crap items and not that one amazing item they keep on showing as the grand prize but no one has actually won it in centuries.
4. Unnecessary registrations
Every bloody game wants you to register on their own sites and Google Play and Leaderboards and what not!
5. Build a character
The number of characters I have built just trying to play a game, I must have added to the population of the country. And there is always something better to equip your character with!
6. No offline mode
A lot of games today think you are disqualified from playing the game if you do not have internet access. This is not always for the game play itself, but to show you those annoying ads.