Microsoft's SharePoint has now topped $1 billion in licenses and is perhaps the fastest-growing product in Microsoft's history. What is driving that growth? The same thing that has driven all of Microsoft's most successful products: Microsoft removes complexity (and cost) from existing markets, as Craig Roth notes:
To a certain extent, the excitement about SharePoint has really been a reflection of disillusionment with existing collaboration, content management, and portal products. The people that are interested in SharePoint - despite already having incumbent alternatives - see at first glance a product that may finally provide easy-to-use, inexpensive, web-based collaborative solutions.
Love them or hate them, Microsoft does lower the bar to computing. Its products can be shoddy (SharePoint is no winner in that department - just try scaling it) but that's a trade-off many are willing to make in order to have something, anything that works reasonably well at a reasonable price.
The question for the market is whether anyone can "out-Microsoft Microsoft" in the collaboration space. I believe we can. My own company, Alfresco, does so, but there are a range of other competitors (Jive, Liferay, MindTouch, Drupal, etc.) that go after the same general problem of connecting people around productivity tasks.
This, I believe, is enterprise software's most interesting market going forward. It will be a chance to compete with Microsoft at the top of its game, on a business problem that will persist and be exacerbated by a recession.
Disclosure: I am an employee of Alfresco, an open-source collaborative content vendor, and am an advisor to MindTouch.