Display and ScreenI was a bit skeptic when the new iPhone 5 had a 4-inch screen. However, since I've also been using Android devices (I am a web and mobile developer) a part of me has grown to like a bigger display. While I find a 5 inch (and larger) display very unpractical I would gladly see a 4.5 inch iPhone 6. However, I would certainly dislike anything larger than that. If you want a larger screen than that, get an iPad or something! Seriously!
Improved Camera OpticsA lot of people criticize Apple for not playing the megapixel game and for offering 8 Mpx on their latest devices. Personally, I never cared for the actual resolution of the photos and this is what I use my digital camera for. Instead, I would like to see the quality of the camera optics improve. Photos in low-light condititions could be better and the new iOS 7 should bring in some powerful new software for the camera app. Higher quality image stabilization is also a feature I look forward to seeing on the new iPhone 6.
Better Battery LifeI cannot say that the battery life of the iPhone 5 was horrible, but this is a general complaint I have about any smartphone. I just want batteries to get better and for the software to be better optimizedso that it doesn't burn so much power. This is an endless request, but so far Apple has satisfied my wishes.
Better WiFi & NFC
The new iPhone 6 needs an upgrade to Wi-Fi 802.11ac but Appleis probably waiting for the IEEE to approve the new WiFi specifications later this year. Since the latest MacBook Air has this tech we can be sure that the new iPhone will also follow. As for NFC chips, I personally don't need it but it is one of the features that a flagship such as the iPhone 6 should have.