There have been attempts to offer this kind of feature to classical operating systems, but the scripting systems have never had access akin to Greasemonkey's in the browser. Even the most sophisticated scripting tools, like AppleScript, access the code only through an API. Purists may not like someone messing around with their variables, but they don't have as much fun. This is why some of the cleverest tools are available in the browsers.
Browser as ultimate OS reason No. 5: Multiplatform simplicity and mutability
There will be some who point out, quite correctly, that not every Web page looks good on odd-shaped screens. Some websites look like a car hit them when they're viewed on a tall, thin mobile screen. The elements are scattered everywhere, and nothing lines up. Something went wrong with the float styles.
Those problems are fading as Web designers learn how to plan ahead for the wider range of browser-based opportunities that are appearing. They're learning to be flexible and elegant so that the content can flow easily regardless of the device on which it will appear.
Browser as ultimate OS reason No. 6: A clean abstraction layer
The Web's open source foundation encourages rapid evolution of best practices for design. There are still some corners of the tech world -- say, Apple -- where a few designers insist everything be done their way using their native framework. But for all of Apple's success in wielding Vader-grade control, it can't beat the Web. Every innovation created for iOS is quickly imitated and rolled into the major frameworks that dominate the HTML5 world.
The browser's democratic populism lets the best ideas bubble up as everyone constantly experiments. This is in large part due to HTML and CSS, which have evolved into a clear abstraction layer that separates concerns, making collaboration easier for everyone. Designers and programmers can target their specific layer and, where possible, bring in predesigned libraries and frameworks to leverage the work of other designers and programmers in their own code.
Browser as ultimate OS reason No. 7: Better sharing models for libraries
Libraries have always been one of the biggest sources of headaches for programmers. One application on the computer wants to use Version 3.4.666 of a library, and the other wants Version 3.4.667, but the operating system can only find the wrong one. When things don't align, some call it "bitrot" and others call it "versioning errors." Either way, everyone loses.
The browser world offers a better way of distributing libraries. Many Web pages link to a few centralized copies of popular libraries like jQuery or Dojo. Instead of using their own version hosted with the website, they link to a central version offered by one of the Web infrastructure companies like Yahoo. These are more likely to be caught by the cache, thus saving the next Web page the time of downloading this version of the library.
These centralized versions are neatly numbered. If two Web pages use Version 1.9.1 of jQuery, then the cache will do its job. If one switches to a newer version, both will function. The cache might not save much time until they start using the same version again, but the switch works without a hitch.