Using the latest 2.10 snapshot from a few days ago (Aug 11, I believe it was), the SQL Server plugin no longer seems to insert a row in response to a WFS insert transaction. This had been working previously on 2.8 before upgrading to the latest snapshot.
The insert transaction just provides a coordinate, no attributes. The SQL Server table has a featid column defined as a primary key, with an insert identity defined. GeoServer sends a query like this (observed using SQL Server Profiler):
..which makes SQL Server say:
Ideally, because featid has an identity defined, that field can just be left off of the query, and it ought to work fine, according to my understanding. As a workaround for now, just removing the identity from the table definition works, but is not ideal.