You also find tips for the following things: How to open graphics with an image viewing program like Irfanview, how to open websites with Internet Explorer or Firefox, how to create tinyurls for the current url and much more.
The other sections are as helpful, I suggest you simply take a look at the whole article and see it for yourself. Implementation is really easy and explained well so that new and experienced users alike can implement the modifications quickly and without running into issues.
Update: Opera has evolved ever since the customization guide was written, and while most of the tips still work in the latest versions, it needs to be noted that some menus may have a different location now in the browser.
Here is the list of customizations the guide is about:
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