While visiting Warwick, I used an old iMac running MacOS 9 at the maths department to check my e-mail. It reminded me of just how snappy MacOS 9 is compared to OS X. You click the menu bar and the menu appears immediately etc. Why don't they do this in OS X? Is it that hard?
I think this sense of instantaneity is very important to usability and is a place where OS X lets us down most. Having things happen immediately doesn't only save time but also is what we are used to from mechanical devices. Pressing down the lever on a toaster will move the bread downwards immediately, not a split-second later. It is the kind of feedback that is most useful as we don't have to wait for it. With instant feedback we can almost 'feel' that we're doing something, having it with a slight delay will need much more attention as we actually have think about what we're doing and match up the feedback/ result with the original action.
Thus, while the delay in opening menus in OS X may be tiny (and sometimes is quite lengthy in fact), it still feels quite large.
There are worse delays as well. Say, if the little transparent onscreen display for the volume got swapped out, adjusting the volume will be a pain as you don't have any feedback for a whole second or so. Similarly it can take perceived eons for the Finder to play the 'trow away' sound after you've dropped something in the Trash. A lesson for the engineers should be: If things need to respond to the user make sure they do it without any perceivable delay. In particular make sure they don't have to be loaded from the hard drive first.
It seems that this feeling of instantaneity is what makes little one-purpose 'appliances' frequently to be more useably and preferable to computers. You just press the button and things happen immediately.
One example for this are cashiers. Peeking over the shoulders of checkout clerks, I gained the impression that the software in the cashiers is very sleek and efficient and it doesn't get in the way of doing the job, causing unwanted delays etc. Until recently, that is... It seemed to me that the software they use at Sainsbury's looked differently. Graphics were fancier and there seemed to be delays as well. Bad.
Another place where this should be important is cars. To operate things, possibly while driving, user interfaces must be easy to understand, easy to use without looking and not distract your attention from traffic, say, by waiting whether what you did actually changed something. That is why having easy to press push buttons where you can feel that you've pushed them are good, where as little touchy things are bad. Similarly, car radios shouldn't have buttons labeled '+' and '-' to adjust the volume but a round turning knob as you can't find those buttons as easily as the larger knob without looking and you don't have the same reassurance that you've actually done something as you have when turning the knob.
To make things worse, looking at my brother's car magazine, I got the impression that many posh cars tend to have menu driven touch screen consoles to give the driver access to all their features. I will not have to elaborate how wrong – and potentially dangerous – this is.
Received data seems to be invalid. The wanted file does probably not exist or the guys at last.fm changed something.