Capped inventory forces you to destroy items or upgrade inventory. Also, having small amounts of left over premium currency.
"You need to spend the in-game currency of "gems" to increase your inventory. You can earn gems slowly without spending real money, so I have gradually increased my inventory for free over time, but it tries to encourage payment"
"You can only keep a certain amount of emotions in your inventory, and you can't collect more of the same emotion until you spend what you have. but you can only visit another planet if you have the incubator going. which you can't do if you can't collect the emotions already done incubating without having to pay for another vial or whatever."
"The limit of 200 coins, which means you can only play a song a day, or 2 songs every 2 days. You can pay for VIP."
"Synchronization Device - The game has a synchronization device that equalizes the level of the characters in said "inventory," depending on five of the player's highest-leveled characters. The synchronization device requires 500 gems, albeit it not requiring real money, maximizing the number of slots available would require money at this point in time."