This.
While the "things I miss about my Blackberry" list is a relatively short one, this feature is probably at the top. A simple, not-distracting-to-others, non-vibrate type notification when new emails/texts come in.
This is on the top of my wishlist for iphone 6. LED light with customizable colors would be great. As well:
128 gb option
5 inch screen
Thicker, with twice the battery life
Other than that, hardware wise, I can't think of anything else that I really really want that apple would ever implement (SD card and removable battery come to mind as nice options, but don't see that ever happening).
Software wise:
Customizable control center, notification center, icon layout, icon size, number of rows, etc. Ability to run two apps at once (top half of screen for texting, bottom half for email, or whatever) would be great. Throw in quick reply for texting, and maybe the ability to forward texts and voicemail to an email address and there really is less and less reason to have a jailbreak. Oh, and allow a different pic on each screen. That's a nice feature too.
I usually upgrade every two years, but I would grab a new phone this fall if they implemented a bunch of these features. Probably would do it just for the hardware related ones I mentioned above even if they do nothing software wise.