I'm making a 'free2play' mobile app, and it will have a premium currency which can be bought for real money...in this game, I want to incorporate some kind of chance mechanics that utilize the premium currency.
For example, let's say the player can spend premium currency to open a special chest. When the chest is opened the player has a chance to get nothing or get some items.
Is this in itself a "gambling mechanic" because it's based on chance? Would that make it illegal for me to include this feature?
Which country? When you ask any legal questions you should state which country you are from, laws differ
To my knowledge this is considered illegal only in Japan and only since very recently. In other countries it's not considered gambling.
And if something like this was ever made illegal you could notice it by observing the TV news, since they would tell you about Facebook and Zynga shares fall :D So it would be hard to overlook.