off the top of my head, expect I'll add more later:
Web interface to work on other browsers/platforms (eg Mac+Safari)
Ability to display an extension's number on screen (on likes of 5312/5324, the 5330/5340 do it anyway).
Ability to setup an analog extension/ONS, with a pre-specified prefix, eg 9 for outside line (for say fax machines).
A way to have a visual indication of day/night mode (eg, you can have a call forwarding key which lights up when active, how about one for 'Night 1')
Currently, automatic switching from day/night, doesn't take into account weekends (so you need to manually handle this?)
Last I saw, the call history application on the 5330/5340 (full screen), couldn't return calls, as it was missing the 9 prefix (and didn't pay attention to 'Outgoing external call prefix for apps' in 'System Options Assignment'
In the web interface, show more than 5 lines/phones/etc at a time, if there is space to do so
Coming from a Cisco background, where you can access the configuration as a text file, export it, save it. And then, compare how it was on different dates/between systems, that is something I miss. Being able to export the settings to one big text file would be good. I'd partly started to be able to do that when I discovered formprint, but, we found some settings which come out differently in formprint, to what the web interface shows, and Mitel said: don't use formprint
If parking multiple calls, on say a 5340, it would be good to show the caller's number beside each button (eg on the LCD screen)
A way to be able to deassign the MAC address, from an extension, from the phone in question would be good (if permitted by the class of service). eg, if you assign the wrong extension to a phone, you can over-ride it, by going to the proper phone, and doing 'Set Replacement Access Code'. But, if you want to swap an extension between 2 phones, you can't do that, so you need to physically swap, or go into the web interface and clear the MAC address. Sometimes it would be nice to be able to return each phone to: USE Super key TO SEND PIN, from the phone
an option for call timer on phone display to run minutes:seconds not hours:minutes
Glad that NTP has finally arrived.