I don't know if SO is using everything that comes with MSSQL, but it's a lot more than just a RDBMS. SSIS for example is a full workflow/job execution environment fully integrated with MSSQL. When I did a similar analysis to move off of MSSQL in the past, there is a lot more to it than just swapping out databases.
SSIS, MSAS and SSRS are minimally functional products that exist from stopping MS customers from looking elsewhere. If they are adding value to you (instead of just being demanded from the high up and never used), it's better to look at the alternatives, some times even if you are already paying for them (that one is a difficult decision, as the others are kinda expensive).
Anyway, the Microsoft implementation of data workflows is so stupid that you are better of with manual dblinks or creating the entire thing in a general purpose language. Their largest competitors share a lot of the same problems, so I'd say that workflow software is just an excuse to hire low-paid professionals and a real drag on any working development team.
In our case we were also using MSAS and SSRS.