* 4.0-4.1 inch screen - larger becomes unusable/uncomfortable for one handed use and/or you have to sacrifice too much pixel density.
* Higher resolution than 480 x 800 - something approaching (or exceeding) retina display density
* 8MP backlit sensor camera with f2.2 lens and great software to back it up
* Non-pentile SAMOLED+ display
* Something like the design of the Lumia 800 - doesn't have to be identical, but has to have some individuality to it. Not just another black rectangle that looks like they skimped on build materials (Samsung) or just retooled the half metal, half plastic design again (HTC)
* noise cancelling second mic
* phone must have an equalizer to adjust bass/treble for music (why, Nokia? Why have a great music app and NO EQUALIZER???)
* dual core for a relative level of future-proofing and to give the phone the horsepower to do things such as screen mirroring
* absolute MINIMUM 1800 mAh battery
And for the OS:
* some form of notification centre
* more accent colours - ideally custom, but failing that, just more of them
* shortcut toggles on start screen
* info section to show how much data a live tile uses so you can make a decision as to whether you want to use it or not
And the only other thing I'd like is for MS to properly open up the OS so devs can take full advantage of the platform and we can have some games that don't look like 2-year-old iPhone ports. I have a brand new, top-end smartphone for this format. I want to be playing the equivalent of Infinity Blade 2, not the millionth reworking of Doodle Jump.