Ballmer's focus on numbers might have been a bit over the top. However, given that numbers increasingly define our successes and failures, having a strong basis and fondness for looking at and understanding numbers will separate the successful executives from those who fail.
Select People for Loyalty, Not Competence
This is one mistake Ballmer made. I'm not suggesting you select incompetent people. Instead, you should value those who can demonstrate loyalty and team-playing over those who are more competent but possess massive egos and don't play well with others.
The silos that crippled the company and created many of Microsoft's biggest mistakes stemmed from the inability of really bright people to work with their peers. You can blame some of this on forced ranking, which pits employees against each other, but a lot of it happened because being smart seemed to trump playing well with others. The end result: A lot of smart backstabbers.
That's not a skill you want to promote -- particularly if you're the top guy with the back that's most likely to be stabbed. This would be like an assassin hiring underlings who are very focused on upward mobility; his life expectancy would be very short. You want people to have your back, not stick something sharp in it.
Validate Your Information
Ballmer's focus on numbers actually worked against him at times. That's because he often received corrupted numbers designed to drive a particular decision or simply give Ballmer the answer he wanted. The old term "garbage in, garbage out" applies here; too much of what Ballmer was fed was inaccurate, which led to a series of disastrous decisions.
You have to constantly make sure that information is accurate. Don't work against this by shooting messengers or letting executives get away with trickery. CEOs in particular can find themselves isolated by direct reports who either want to manipulate them or simply make them happy by falsifying or limiting what the CEO sees. This kills companies and makes even the best CEOs ineffective.
Don't Isolate Yourself
Before becoming CEO, Ballmer talked to people both inside and outside Microsoft to gain perspective. After becoming, CEO he allowed himself to be isolated by those who apparently wanted to better control the outcome of his engagements. I saw this happen to the only CEO who was ever fired from IBM, John Akers.
There are always those who want to either protect or control people in power by limiting their access. If that's allowed to happen, bad things go unnoticed for too long. At best, the executive is ineffective; at worst, he or she fails and takes the company down, too. One advantage of maintaining contact both inside and outside the company is that it showcases areas where you're being misled while you still have the authority to take action.
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