Transactions are one thing I want the most, and that's not going to happen on S3. Sure, I can reinvent them by hand, but the point is I want that baked in.

Yeah, closest thing there is MS-SQL FILESTREAM, but even that has flaws and severe limitations... you can do similar transaction implemenations for binary data storage in any given RDBMS, or do similarly via behavior to read/write to filestream along with a transactional row lock that corresponds to the underlying data. But that gets its' own complexities.