He'd stop palling around with Google
Apple's relationship with Google has cooled in recent months - the hoo-hah over Google Voice shows how the firms are becoming rivals, as does Eric Schmidt's departure from the Apple board - but they're still joined at the hip: that search box in Safari is a nice little earner, and Google Maps is in everything from the iPhone OS to iLife.
Ballmer, of course, isn't a big Google fan, and famously – albeit allegedly – chucked a chair across his office while bellowing that he was going to "f***ing kill Google".
While Ballmer describes the story as a "gross exaggeration" and says "I have never, honestly, thrown a chair in my life", there's no doubt that Apple's still-cheery relationship with Google would come to a spectacular and ill-tempered halt.
Ballmer is many things, but he's not a creator. Before taking over Microsoft his role was to take care of business so that Gates could do the vision thing; today, Ballmer's the boss but Ray Ozzie's the man with the vision.
Unlike Tim Cook, Ballmer doesn't play a big part in product design decisions. "Ballmer is a very talented business manager, not a visionary," Enderle says. "He would turn over the responsibility for products to individual line managers."
However, that could easily backfire. History suggests that putting too much trust in line management isn't always the right thing to do: the dying days of the Sculley era saw empire building with managers pushing their pet projects and trying to protect their own backsides rather than thinking of the bigger picture.
Apple works because of Steve Jobs' control freakery, not despite it.
He'd buy Jonathan Ive a diamond-studded throne strapped to the back of a unicorn
Apple's success is largely due to the combined smarts of Jonathan Ive and Steve Jobs; with Jobs out of the picture, Jonathan Ive could demand pretty much anything and Ballmer would have to provide it.
He'd make too many products
Ballmer has been Microsoft's CEO since 2000, which means that all the things people mock about Microsoft – umpteen different versions of the same product, the Zune and a love of digital rights management, to name but a few – happened on his watch.
"Microsoft tries to optimise for every possible price and variant, and they don't realise that that's a market killer," Enderle says. Why have one version of OS X when you can have OS X Home and Student, OS X Basic, OS X Premium, OS X Ultimate, OS X Professional, OS X… you get the idea.
Ballmer's comments on the iPod in a recent CIO Magazine interview show exactly where he's coming from: Apple "may have a cult following in the music business, but we've got about 97 percent of PC users using our stuff. 97 percent may not constitute a cult. But I wouldn't trade that for a cult."
Apple wants people to love its products; Microsoft doesn't really care if they love them or not, as long as they buy them.
"To an extent Ballmer would try to recreate Microsoft, but he's not Bill Gates," Enderle says. "I don't think it would work any more than Steve Jobs would work at Microsoft."
Ballmer might boost OS X's market share, but he'd have to sacrifice Apple's soul to do it.
Liked this? Then check out What if Steve Jobs ran Microsoft?
Sign up for TechRadar's free Weird Week in Tech newsletter
Get the oddest tech stories of the week, plus the most popular news and reviews delivered straight to your inbox. Sign up at http://www.techradar.com/register