I just asked Why are off-topic migration options so limited? Should more specific SE tech sites be more visible?, and it made me wonder if we should have an ability to search across multiple, related Stack Exchange sites.
The very presence of a migration feature indicates that sites will have closely related, and sometimes, overlapping content. For a real example, compare the results of the queries for "IIS Basic Authentication" on Server Fault and on Stack Overflow.
Users might not know they need to search across multiple sites to find what they are looking for, and honestly, they shouldn't really have to. I think a consequence is that users will end up conducting their searches on Google or Bing instead of on Stack Overflow. After all, if you know a little bit about Googling, it's pretty easy to query multiple StackExchange sites exclusively from there. (see an example for googling "IIS Basic Authentication" exclusively on both StackOverflow and ServerFault).
Is there a reason that we don't have stronger integration between related Stack Exchange sites with respect to search?