Capped inventory forces you to destroy items or upgrade inventory. Also, having small amounts of left over premium currency.
"Limited to upgrading one building at a time. Paying opens extra upgrading slot. Troop and hospital sizes locked behind building levels."
"500 item inventory limit for all items. Players constantly have to break down useless equipment or use food."
"You can only have a set amount of space for your cards. Irrelevant if you just play songs, but a pain for collectors. It technically takes premium currency, so it's a YES, but it's so little you don't actually have to pay."
"You have to spend Catfood (premium currency) to increase the amount of available unit lineups."
"It does have a capped inventory system, however you can expand that through the in-game currency. If you run out of in-game currency, you can choose to buy more."
"Yes it sure does! And it’s quite expensive the more inventory you buy the more expensive it gets"
"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."