Of course this article won't include things that are already mentioned by Nokia-Microsoft such as ovi maps etc.
1. Optimized for multi-tasking
S^3 devices even with less powerful processor and small amount of memory, can handle multiple load of running application. This is one of the reason why I always buy symbian smartphones. I can't close my messenger, browser, music player playing in the background while i'm playing games just when someone texts me.
This is a nifty feature, it saved me a lot of times such as:
a. Copying contents from other devices without needing a pc or laptop
b. Recharge my other phone
c. Recharge my bluetooth headset
d. It can even power USB fan
3. Memory card support
standard 16gb on board storage are not enough these days.
Its HD era. This is a must.
5. Easy to customize
Create personal themes, re arrange home screen widgets
6. Powerful file manager
Easily manages file , supports also compressed zip files.
7. Multimedia extras such as image and video editor, web tv, flash (im not sure though if there is a flash support on windows phone 7 already), internet radio
Smartphones must have copy and paste (plus cut too)
9. Mass storage and bluetooth file transfers
Windows phone 7 does not support this yet, im not sure with bluetooth file transfer though. I need mass storage to just simply copy videos or pictures or use my phone as a backup of files.
10. FM transmitter
This is hardware specific though, is not a necessity for most people but me. I've been using this on my friend's car that doesn't support bluetooth music streaming. and also on cars that has bluetooth streaming support but is really hard to pair with bluetooth devices.
How about you, what features do you want to see?