Many on the Yahoo! Developer Network will likely be unaware of things like Live Search, Office Live or Silverlight, or will have ignored them because they were talked about on Microsoft sites and blogs, or viewed as being "too Windows or .NET centric" and not relevant in a Java and open source world. Yahoo! already makes a number of .NET resources available to its community, mostly around email.
Maintaining and successfully developing the Yahoo! Developer Network, would be a major challenge for Microsoft as it requires a philosophical rethink.
First the product focus would need to shift. Yes, Microsoft has excellent developer outreach through MSDN and the company has been going more open source in recent years through things like CodePlex. Still, though, CodePlex lacks the impact of Yahoo! while MSDN is pretty much based around specific products and the .NET Framework.
Next, Microsoft will need to decide what is going to happen to Yahoo! properties that compete with Microsoft - portal, mail, IM, collaboration, media and the ads-serving engine. Will they continue or will they be "migrated" over time to a .NET-centric framework, runtime and tools? If the latter, Microsoft will need to make resources and APIs available to those currently using Yahoo! properties.
Ultimately, Microsoft will need to put community ahead of corporate agenda - and that's a stretch. The company will need to get into the Yahoo! mindset of running an academic-style program where stuff is given away to encourage growth, where there's no marketing or product agenda that says: sure, you can do this on Linux, but - hey - it'll run so much better on our product.
Among those considerations, Microsoft will need to move away from its corporate endorsement of WS- that it once pushed for building web services - in favor or REST, which is finding genuine grassroots success as an alternative to WS-.