Bill Gates recently sat down for an interview with CBS' Charlie Rose to discuss a number of topics ranging from his life after stepping down from his day-to-day activities at Microsoft almost five years ago to his work with the Bill and Melinda Gates Foundation. But perhaps most interesting of all was his opinion on Microsoft's entry into the mobile industry and how his successor has done since his departure.

When asked if he was happy with Ballmer's performance at CEO, Gates said he and Ballmer were two of the most self-critical people and there were a lot of amazing things that Ballmer's leadership accomplished with Microsoft during the last year. Gates specifically mentioned Windows 8, Surface tablets and Bing as being key to the future but ultimately, he said it isn't enough.

Gates said the two were not satisfied that they have done enough in terms of technological breakthroughs. Looking back, he pointed out that cell phones were one of the things that Microsoft didn't get out in the lead early on.

He said they didn't "miss" phones but the way they went about it didn't allow them to take the lead. When probed to elaborate further, Gates said it was too complicated to discuss but affirmed that it was a clear mistake on their part.

If you recall, Gates stepped down as CEO of Microsoft in January 2000 and established the Bill and Melinda Gates Foundation later that year. His last full-time day at Microsoft was June 27, 2008 although he remains with the company he found as a non-executive chairman.