Microsoft (MSFT) CEO Steve Ballmer will exit his post within 12 months. Over the past five years or so, Ballmer has faced plenty of criticism from investors, customers and partners — especially as the world went mobile with smartphones and tablets from Apple (AAPL), Samsung, Google (GOOG) and others. So what were Ballmer’s 5 biggest mistakes? Here’s the concise list.

1. Zune: Let’s build a MP3 music player just as the market is peaking and roughly 12 months before the entire world jumps on the next bandwagon: Touch-enabled smartphones.

2. Windows Vista: Let’s build a bloated, confusing PC operating system. Let’s make it fail to work with older applications. And let’s launch it right as the iPhone arrives.

3. Surface RT: Let’s launch our own under-powered tablet that doesn’t work with traditional Windows applications. Let’s make it really expensive. And let’s alienate our PC partners.

4. Succession Plan: Let’s reorganize Microsoft in mid-2013. But let’s make it really confusing about who may be in line to succeed me as CEO.

5. Partner Program: Let’s launch new services and products like Office 365 and Surface — but let’s make sure there are little to no partner program details at launch. And let’s wait about a year for the products to mature before we really start to offer comprehensive partner programs.

Ballmer has made some big mistakes to be sure. But let’s not call his CEO run a failure. He also had some big successes — which a lot of critics overlook.