Thunderbird 2 bad interface decision: Ctrl+Shift+M
What I see as the primarily, though difficult, goal of user interface design is making the interface as easy to use as possible without sacrificing functionality in the process.
I acknowledge that there's always discrepancy as to where the boundaries lie; however, I'm also of the opinion that some UI decisions are just plain dumb. A significant part of usability is appealing to existing expertise in the user base — this eliminates large fractions of the learning curve straight out of the box.
Some UI choices, like keyboard shortcuts, have lots of inertia. This inertia is not only rooted in history, but also in the muscle memory of users. When my brain thinks "paste", my fingers almost automatically respond with Ctrl+V, as it is still the standard paste operation keystroke. Many people also recognize Shift+Insert to be a valid secondary stroke option, but this is usually in environments where usage of Ctrl+V has historically recognized alternate effects. I posit that this inertia and reinforcement of the Ctrl+V keystroke standard is a Good Thing, as it offloads the thinking process from "what keys do I need to press" to "what is it I want to do" as the keystroke becomes more and more intuitive.
I certainly respect Thunderbird's uniqueness as a mail/newsfeed client, and whatever primary goals they have in UI design have certainly shown with the Thunderbird 2 implementation (it's quite pretty and usable); however, I can't condone their use of Ctrl+Shift+M.
In the two other GUI email clients I've ever used (namely, Evolution at home and Outlook at work), Ctrl+Shift+M composes a new message. Though it's been a few months since my transition from Evolution to Thunderbird, I still push Ctrl+Shift+M sometimes when I want to compose a new message. In Thunderbird 2, this makes the currently selected message magically disappear without explanation.
When this happened to me, undo worked (thankfully), so I knew I hadn't permanently deleted my email. I then looked to Google and realized that Ctrl+Shift+M now performs the "Move / Copy Again" command, via the Thunderbird keyboard shortcut page.
I understand when a UI that departs radically from the functionality of other popular interfaces differentiates keyboard shortcuts as well. This actually serves to establish/reinforce another mode of operation in the user's mind! "These new actions are what I perform when dealing with this radically different scenario," seems to be a strong corollary of Pavlovian reasoning.
Thunderbird, however, looks and functions in a manner very closely resembling other popular GUI mail clients. In disregard of this similarity, it took the existing, inertial interface functionality and made it do something different and potentially destructive. That's far worse than doing nothing.
An aside: I understand that there's probably a way to neutralize that keystroke; however, this is a critique of the UI decision making as it ships "out of the box". Additionally, I've never found altering the hotkeys in Mozilla applications to be as easy as it should be — I've had to do things with unintuitive shell scripts and creating previously non-existent configuration files.