If you haven't already, there will probably be more pressure to update to iOS 8 this year than there has been with similar updates in previous years. Out of the box, you may not immediately notice much of a change after updating from iOS 7 (which, by contrast, marked a major visual redesign compared to its predecessor). But iOS 8 is packed with new features for developers, which will slowly start to trickle into apps over the coming months if they haven't already. There are probably a large number of cautious laggards who would prefer to wait for things to "bed down" before upgrading. But sooner or later, you may well find that your favourite app can no longer be updated without upgrading to iOS 8. In the meantime, most users have surely noticed the flurry of updates arriving in the app store as developers scramble to fix issues that have come to light as a result of the iOS update.
One of my own apps illustrates the kind of subtle issue that developers are having to grapple with. The Utter French pronunciation guide, as the name suggests, makes fairly extensive use of audio playback. Like many, I tested the app using the iOS 8 beta, but one slightly subtle bug did pass me by.
As mentioned, the iOS 8 issue I discovered relates to audio playback. The issue is caused, not so much with the audio per se, but rather what happens after a sound has been played back. At the end of audio playback, the system notifies the app via what in Apple speak is called a delegate method (in effect, a callback to what is akin to an interface method implementation in Java). The system makes a call to the programmer's implementation of the audioPlayerDidFinishPlaying playback. method to indicate the end of playback. Crucially, it passes in a boolean parameter to indicate whether playback was "successful". Subtly, between iOS 7 and iOS 8, for reasons not yet clear, the value of this flag changed from TRUE to FALSE in the cases I observe, despite the fact that audio apparently plays back correctly. Other developers have also reported this issue. The reasons and circumstances remain unclear: perhaps iOS 8 has tightened up on some subtle features of audio files that it now deems to be "incorrect". Or perhaps it's just a bug that will be fixed in a subsequent maintenance release of iOS 8. But the knock-on effect for my app was that it took the FALSE flag is a signal not to play back the remaining items in a list of audio files queued for playback.
The workaround-- currently in review and hopefully to be released shortly- is simple in this case. But it illustrates how a very tiny change in the behaviour of an API call can have a knock-on effect. Multiply this kind of subtle change by who knows how many system functions and we start to see why developers are scrabbling to release updates and fixes for iOS 8...
Despite this, my advice to most users would still be to update iOS 8 fairly soon. The stream of daily updates in the App Store indicates that developers are getting on top of the issues, and it won't be long before a new app or update comes out with an iOS 8 only feature that you want!