I just installed the RTM of Office 2013 Pro and with most everything installed it was under 3GB. That leaves the OS (w/recovery, etc.) taking up around 13GB by my count. Round it down to 10GB and that's still a lot compared to the competition. No doubt RT provides Office where the others don't but to loose more than 10 times the storage as the nearest competitor just for "Windows" with Office IMO is going to be hard for most consumer to accept. With the Office web edition and Office mobile for iOS and Android coming next year it's going to make that space issue even more of a glaring problem.
That said the amount of memory free on a device is just one factor of many that buyers consider. There are other things such as availability of applications, reuse of existing peripherals, and performance that are probably more important.