I've been playing with Extensions and Today screen widgets in iOS 8 on my iPad, and I'm just fine still using my 4s with iOS 7. Some iOS 8 features add convenience, but a javascript bookmarklet to add an article to Instapaper works just as well as an Instapaper extension. There is currently nothing in iOS 8 that I feel I have to have on my iPhone.
The only app whose release I "dread" is Drafts, because I know that it will be iOS 8-only and will sync with Cloudkit. As long as I keep my iPhone as-is, I will not be able to upgrade to Drafts even on my iPad if I want to sync notes between my devices.