The high usability example: Gmail

Why is Gmail an example of high usability?

This happened in two steps, by sure. I imagine it like this:

First step, the idea:
The first step was a positive one, about thinking on behalf of the users, the inception of a useful product.
Remember: before Gmail we used slow webmail, with very little alloted space. For example I used Netscape mail because it offered 10MB, twice as much as the others did!
Also, web mail was cumbersome and slow, made with traditional HTML forms that took ages to reload completely making you lose the focus.

On the other hand we had the desktop mail, with enough space to store messages and with quick interaction. But usually installed in a single PC, unreachable from elsewhere.

The Gmail idea was to question this status quo and seek a solution providing the advantages of both web mail and desktop mail:

• reachable from anywhere,
• quick response,
• lots of space to store messages,
• and the innovative "conversation" newsgroup-like organization.
  

Second step, the implementation:
This second step was a "negative" one in that it was about not doing wrong.
Although the developers (I imagine) were conscious that they were challenging the model, they refrained from adding the many features that surface in such circumstances, coming out with a really clean user interface.
Those who are not developers have to imagine how compelling is to show the world a feature that just appeared in one's mind. Whatever excitement you imagine, please double it.
Yes, the team was led by someone with really strong ideas! Perhaps a woman, because women are much more resistant than men to the gadget kind of feature.

The result was an interface that anyone could use without special training, just having operated another email client (even the infamous Outlook) was enuogh.

Wrap up
The first step was to have a good idea, and the second was not to spoil it.
Gmail changed the world. After it's success all other web mail services had to catch up in a hurry, some of them did not resist the urge to add frills and came out with heavy UIs that their users complain about.
Gmail is so good not because it's Google's mail approach but because the team leader did not let features creep into the devilered product.
As I like to say, usability is the lack of defects. One can not add usability but take defects from the product.
Best of all, an application can be born with no defects, strictly sticking to a minimalist design plan. This is what returns the best results.