I was confronted with this question just yesterday and it really took my attention. It is obvious that most experts (or who ever calls himself that way) see Microsoft as a player in this space.
I must admit it is hard to get the whole picture pinned together. I will try to shed some light why Microsoft certainly rides the wave...
There are several pillars and a technical basement.
Regarding the technical basement we over a whole stack: Starting with a server OS, a database up to a very stable and scalable implementation of a web framework (not to forget the AJAX framework on top of it (ATLAS)). But this was the easy part. Because we are a platform company.
Beside this the next web is more a business model than a technical thing. Microsoft has been and carries on to be a partner driven company. So we offer levels to adopt and base your very own solution on throughout the stack. going higher within the stack there is something we call SaaS (Software as a Service). This is the part which focuses on Small and Medium Businesses as users and which can really benefit from the monetarization models coming in from the next web. Our idea here is Office Live based on SharePoint.
Beside this there must be a portal focussing on active end users with the capability to be changed and adopted. We call this Windows Live.
Then we need something build for the typical consuming end user. Somebody searching for a portal showing things in a easy manner, not necessarily adoptable but with all the services needed. We call this MSN.
Very new is the possibility to include your game console into the play. Having a gaming framework spanning from xBox to the PC is a cool thing to offer. We call it XNA and xBox Live.
Oh, not to forget we need baseline services like advertising or identity management. We will provide those services, too. Especially in the identity management area we learned a big deal regarding passport. So CardSpace will change the game here.
If asked "Are you complete yet?" I would answer "Will we ever be?". The nice thing regarding the next web is that it is just starting yet. But to be honest I am pretty confident we are well prepared.
Greetings from Budapest...