Because by some other standards, such as Fitts' Law, it's a good move - it gives the menu infinite vertical size.

Also, it is an effective way of conserving vertical screen real estate. With the increasing prevalance of widescreen displays, vertical space is (relatively) scarce. This is particularly true on netbooks, and even on full-size laptops. Merging the menu with the top dock gives you an extra 24 pixels or so of vertical space.

A couple of years back I switched from Windows to Mac and I can't say I really noticed any downside to the single fixed application menu. The gains due to Fitt's Law however are noticable and I when using a Windows box I find the menus to be annoyingly small click targets.

I do miss the discoverability of Windows ALT+letter menu accelerator key presses though.

You might find Cmd+Shift+/ useful then. It allows you type menu commands on keyboard, see where they are in the menu, and discover new functionality.

The default shortcut to focus the menu bar is Ctrl-F2. I change it to Ctrl-2 and frequently do things like "Ctrl-2 F <Return> O <Return>" to open recent files. For leaf menu items I set per-app keyboard shortcuts in the Keyboard pref pane so I don't have to do the Ctrl-2 song and dance to find things I use all the time.

There are some undiscoverable ways to use the keyboard on OS X. In many alert dialogs you can hit Cmd-[first letter] to select that button, e.g. Cmd-D for Don't Save, and Cmd-M for Move to Trash. They don't underline the letter because it's ugly, but once you know about the convention it doesn't matter. I like the way Windows displays the underlines when alt is pressed.

Your first point is very useful. Still way slower than the Windows way but way quicker than mousing it.

I already knew the alert dialog trick. It doesn't work consistently though which is rather annoying (Apple's own apps rarely seem to support it)

What about applications like Firefox 4 that don't require full-blown menu bars? What about applications that want to try out alternative user interfaces (ribbon etc)? What about multi-monitor setups? What about high-resolution (1080p or higher) monitor setups? What about high-res multi-monitor setups, where you might have to move the mouse over 4000 pixels to get to the menu you're looking for?

As a high-res monitor user, with proper mouse acceleration it's really not any harder to hit a menu item that is anchored to the top of the screen than it is to stop the mouse on a menu item 200px away.

The only issues I've seen are when I have a non-rectangular monitor configuration.

Proper mouse acceleration is a poor poor solution for the parents problem. I can't (physically can't) use a mouse that moves faster than a certain speed.

Firefox 4 removing the menu bar is a one-off attack on the same problem. Better to solve it once than require each application to come up with its own fix.

I'm not sure what you mean. The space that Firefox 4 saves by removing the menu bar would just be wasted at the top of the screen.