I forget the name, but I had something where if you held down the windows button, the entire screen would be covered in a grid of customizable buttons. You then dragged to whichever button and released your finger. This was accessible from any app at any time, and is the fastest shortcut system ever... Just a flick of the thumb and I could toggle airplane mode, or start the flash light, etc.
Also I miss xdaUC for obvious reasons.
This isn't really an app, but I miss the internal file picker of WM6. It had an address bar which supported all protocols. WP7/8's file picker is a HUGE regression. For example, if I had a normal picture viewing app, i could browse to My pictures... Or type
http://google.com/logo.jpg and it would be fetched. Or \\LAN-PC\shared-picture.
A simple video player would not have to support network functions because the system file picker handled that automatically. The video player TCPMP was aware of this and offered controls for how big the buffer should be, and etc, in the event the file was not sourced locally.
Now it falls to each and every developer to implement each and every network protocol (and they never do for most apps), and the file picker necessitates that every app copy every file into isolated storage, which causes long wait times and effectively requires you have double the file size in free space (opening a 4GB video will result in a long wait while it copies 4GB into app isolated storage).