The look of things
One of the reasons Microsoft will be able to stand-in against open source is the importance of interface. To the typical business software consumer, the user interface is the software. They don't know or care how it operates under the hood. It is very difficult in a diffuse and not-for-profit project to get someone to do the hard slogging to refine and improve the user interface (and let's not even start on documentation). Although making a pretty good interface is possible just by following solid user interface principles, exceptionally usable software requires investment in usability testing and refinement. This kind of interface design and evolution is not something that open source projects have proven to be good at.
Maybe there is a business model in there somewhere? But I'm not sure you could get past GPL to make your interface proprietary and thus worth investing in.