I've spent the day at Microsoft's unveiling of Office 2013 at the Metreon in San Francisco. This product has been years in the making. It was conceived before the iPad hit the shelves, and its improvements are largely PC-focused–Excel, Word, and PowerPoint deliver richer and more fully-featured experiences on the PC than ever before. It's a product that has adapted to the multi-device lifestyle, with user-based subscription pricing (Office 365) and cloud-streamed Web apps (Office on Demand)–but the PC is still the star, and tablets are an afterthought. Office does have a mobile strategy, but that's explicitly not the focus of this event today. Even Microsoft's own Windows 8 platform won't get native Metro apps for all the Office programs at launch. (The version of Office that will be available for Windows 8 and Windows RT at launch is touch-optimized but won't use the Metro UI, except for Lync and OneNote, which will be native "Windows 8-style" apps.)

Office is a $20 billion business, and Office 2013 is the best version of Office yet. It will sell millions of licenses to consumers and enterprises (Office 2010 has sold more than 100 million copies, and that doesn't include the millions of users who use pirated versions of Office). But products at the peak of their success can still be vulnerable to disruption, and Office 2013 certainly is, especially to competitors who put mobile first, and who deliver less-good experiences for cheap or free.

Productivity apps for post-PC devices are flourishing. Most, like the Slideshark, Prezi, and IdeaFlight presentation apps, are complementary rather than cannibalistic to Office–that is, they are useful because PowerPoint is so ingrained in consumers' and workers' productive lives. But others–notably Quickoffice, which was recently acquired by Google–are very, very dangerous to Microsoft, precisely because they are easy to dismiss as not as good as real Office. It's true, the product experience of Quickoffice is nowhere near as rich as Microsoft Office–and that's exactly why it's disruptive. It's a lot cheaper, and it works across devices and platforms. It's a classic example of Clayton Christensen's model of disruptive innovation: a process by which a product takes root at the bottom of the market, and then displaces competitors higher up the food chain. Now that it's owned by Google, Google Docs + Quickoffice will have a much stronger play on tablets and smartphones, and the little competitor that was easy to ignore just got a whole lot more powerful.

There's a lesson here for all product strategists: Your best product–the richest in features and user experience–may be your most vulnerable to competitors who do less, cheaper, but with more convenience for the consumer. Today Microsoft raised the bar for the Office experience on PCs. But in a post-PC era, a PC-first strategy is risky. The PC built a fortune for Office, but post-PC devices still look like a vulnerability, not a strength, for Microsoft Office.