Well, let's start, I don't think this list is exhaustive.
Consumers:
- Unless you carry two mobile devices, you can't use several apps that are iOS-only or Android-only. This is because Apple refuses to support Android apps, and Android cannot easily support iOS apps because iOS is tied to iPhone/iPad hardware (and other tech/legal issues caused by Apple)
- If you decide to only carry an iOS device, your freedom is restricted and you can only run software according to Apple's permission. Also, it's not obvious to the average consumer which categories of software they have been prevented from using by Apple (this includes or included things like a faithful port of Firefox or Chrome, BitCoin wallets, BitTorrent software, games including the confederate flag and so on).
- If you want to run iOS software, you have to pay Apple several hundred dollars more than you would in a competitive market for iOS devices, and can only choose between devices Apple makes
Software developers:
- Reaching a large majority of mobile users requires writing the software twice, due to existence of iOS and the fact that Android cannot support iOS apps and Apple refuses to support Android apps
- Reaching the large market share of iOS users forces you to support Apple's practices, buy and manage Mac hardware, warp your software at additional cost to fit Apple whims, wait days to get updates delivered (if Apple decides to accept them) and pay 30% of your revenue for a service that has at most 5% marginal cost to Apple
- Some software cannot be written for iOS because Apple doesn't allow it, reducing the market for it and potentially making it not viable to develop it
Phone/tablet makers:
- Lots of people only want to carry one device and want/need to use iOS apps (due to lock-in and exclusive software), so the market for new mobile devices is reduced even if they are better in all aspects than existing ones
Society:
- Less innovation happens due to Apple's restrictions on software and the reduced market for non-iOS devices
- Freedom is being curtailed by Apple while offering users benefits for accepting that curtailed freedom, contributing to change cultural norms to a state where it is acceptable for such a company to decide which software and hardware is allowed in the marketplace, and in general what businesses and customers are allowed to do and publish
If Apple were split into three truly independent companies, one selling hardware, one selling iOS and OSX, and one offering the App Store and other online services, most of these issues would no longer be present.
Google is also guilty of several things similar to what Apple is doing, although they are generally less restrictive and it's possible to opt out, and thus less of an issue.