I with you on that!
I'm still wondering how this is going to change (are they seriously taking away the ability to middle click [3rd button] and squeeze [4th button]??):
Correct, Magic Mouse has "Primary Button", "Secondary Button", 'one-finger scroll' (up/down and left/right), and 'two-finger wipe' (left/right only.)
'one-finger scroll' is not configurable, it is solely scrolling, but is similar in function to the scroll ball on the Mighty Mouse (er, "Apple Mouse" now.)
'two-finger wipe' is also not configurable. It solely does certain fixed functions in certain apps, like back/forward in a web browser, and basically the same as left-arrow/right-arrow in most other supported apps.
There is no replacement for "Button 3" and "Button 4" on the Mighty Mouse. There is nothing for 'two-finger wipe' up/down (it just does the same thing as 'one-finger scroll',) and there are no multi-finger taps available; no more-than-two finger swipes available, no gestures available.
With as many sensors as are embedded in the top of this mouse (ifixit's teardown said some number over 100 when it was first posted, now I don't see a number,) I hope the API allows direct third-party use of it.