Friend Invite and Coins through Apps
2 features that are common among apps which would help keep players would be the ability to invite friends through facebook since not everyone has an iPhone instead of an iPod touch which makes an address book useless unless they actually use it on a touch. Secondly the ability to gain coins from partnered apps or some other means then spending money. Don't get me wrong, keep purchasable packages for large coin demand, but atleast allow acouple coins even if it's limited to a download per day in which it would take just under a week to gain enough for a 10 coin item. That's my 2 cents.