Apple announced Monday that iOS chief Scott Forstall would be leaving the company. While most commenters seem happy to see him go, it's worth noting that Apple is parting ways with one of the people who turned it into the world's most powerful tech company.

Forstall was reportedly difficult to work with, according to The New York Times (hey, so was Steve Jobs) and even refused to sign an apology letter penned by CEO Tim Cook over the Maps debacle. News of his departure was met by "quiet jubilation" by some at Apple, according to other reports. And with Jony Ive now in charge of the look and feel of products Apple-wide, some are hoping this means a move toward more streamlined, elegant UIs for Apple and the end of the silly skeuomorphic design that Forstall reportedly preferred, like the fake stitching and leather of so many iOS apps.

But Forstall was about way, way more than just the look of Calendar and Contacts. And to focus only on those would be to ignore some landmark innovations he helped shepherd to market during his 15 years at Apple, projects that fundamentally changed both desktop and mobile computing. Here's a look at his five greatest contributions.

Bringing Unix Support to the Mac ——————————–

Before Apple acquired NeXT and used its operating system to revamp its own, Mac OS was based entirely on code developed at Apple.

Starting in 2000 with Mac OS X, Apple's desktop operating system has been Unix-based. Why is this significant? Many developers were already familiar with the Unix operating system. With a Unix base, coders could use standard Unix commands in the command line, there was greater code portability, and Mac OS X got all of the associated benefits of Unix. For example, Unix is architected in a modular fashion, which can be beneficial when it comes to system security.

Mac OS X Leopard —————-

The Concorso d'Eleganza Villa d'Este is more than a beauty contest for cars. It is an annual reminder that the automobile has yet to be eclipsed as the ultimate expression of the human desire for speed, elegance and sheer mind-blowing cool. Combine the Goodwood Festival of Speed with the Geneva Motor Show, then add the breathtaking scenery of Italy's Lake District and a little petrolhead aristocracy and you've got the Concorso d'Eleganza. It's held late each spring on the southwest shore of Lake Como on the grounds of a 16th-century villa converted into a high-end hotel. There was no shortage of speed and cool at this year's event, the 13th since BMW stepped in as a corporate patron. Such automotive beauty pageants, known by the French name concours d'elegance, started during the years between World Wars. The Concorso d'Eleganza Villa d'Este debuted in 1929 and quickly became the place to show off the newest, best and most expensive cars of the day. World War II put an end to lakeside cruises in coach-built Alfa Romeos. Following a few halfhearted postwar attempts at a revival, European concourses were gone for good. They emerged decades later in the United States at places like Pebble Beach and Amelia Island. What makes the current, BMW-flavored Villa d'Este so special is its embrace of the spirit of the original. Although the focus is undoubtedly on impeccably retouched classics in eight categories, a ninth category features contemporary prototypes. It's a refreshing alternative to the all the cars restored to a level far beyond their original factory conditions. Yes, a 1930s Bugatti or 1950s Ferrari is a breathtaking spectacle, but it's also a museum piece. It is rather more interesting to see what modern designers come up with when they are allowed to dream and play. Above: Concorso d'Eleganza Villa d'Este ——————————– The event is held on the grounds of Villa d'Este, a lakeside villa turned into a luxury hotel. In the foreground are several 1950s race cars and grand tourers like the blue-on-white Maserati Birdcage 63 and the red Mercedes-Benz 300 SL, while you can see several interesting 1960s prototypes in the rear, like a four-door version of the Citroën SM and Giovanni Michelotti's 1968 DAF 55 Siluro Wired staff writers playing with Photobooth.

After beginning his career under Steve Jobs at NeXT as one of the chief architects of the NeXTSTEP operating system, Forstall continued on to Apple to lead development of Mac OS X.

Forstall was fully responsible for Mac OS X beginning in 2006 and was responsible for the release of OS X Leopard in October 2007. It contained more than 300 improvements over its predecessor, OS X Tiger, including support for 64-bit applications and the backup service Time Machine.

Even more importantly, Leopard introduced Photobooth. And we all know how that has enhanced and enriched our lives. (Even Steve Jobs took some time to have some fun with Photobooth).

The Aqua User Interface ———————–

The original UI for Mac OS X, called Aqua. The original UI for Mac OS X, called Aqua.

Scott Forstall spearheaded the Aqua User Interface, the graphical UI that has graced Mac OS X since it debuted at MacWorld in January 2000.

Although the look evolved over the years, from translucent pinstripes to brushed metal to the gradient gray color scheme we've seen since OS X Leopard, two of its key features have persisted: gummy-like buttons (like the ones for closing a program window) and a dock for quickly launching and multitasking between applications.

At its unveiling at MacWorld, Steve Jobs said of the UI, "One of the design goals was when you saw it you wanted to lick it."

A Touch-First Mobile Operating System ————————————-

The iPhone wasn't the first touchscreen device or even the first smartphone. But Apple did pioneer an industry-changing touch-first operating system, iOS, and Forestall led the team that developed it.

Forstall talked about what it was like rethinking the computing experience in his testimony during the Apple v. Samsung trial in early August “One of [the challenges of developing iOS] was everything we dealt with before was based on mouse and keyboard, and here we were changing the entire user interface to be based around touch,” Forstall said. “We had to rethink everything about what big controls would be, knowing where you are in the document, knowing when you reach the list…. Every single part of every device had to be rethought for doing touch.”

Android? ——–

“I will spend my last dying breath if I need to, and I will spend every penny of Apple’s $40 billion in the bank, to right this wrong,” Jobs said when the first batch of Android smartphones landed in 2010. “I’m going to destroy Android, because it’s a stolen product. I’m willing to go thermonuclear war on this.”

Apple has since repeatedly insisted that Google's copied iOS. "You’re stealing all the value we’ve created," senior VP of marketing Phil Schiller said of design copycats in the industry. Even Windows Phone's Joe Belfiore got in on the action at Monday's Windows Phone 8 launch. “[The phone interface] was standardized by Apple and the iPhone and really it was copied by Android,” Belfiore said.

If Android did copy iOS, well then by extension, we have Forstall to thank for the world's two most popular mobile operating systems. Thanks, Scott!