SQL Server 2012 - Dropping Snapshots takes hours
Can you please confirm the Compatibility level of the database you upgraded? (Right-click the database -> Properties -> Options -> Compatibility level). If this is set to 2005, please don't change it without thoroughly testing your applications, etc. as it could definitely break your environment! Just curious as to the value here.

Also, I did some work with a test system and didn't see the behavior you describe. Even if I had an open transaction using the DB snapshot I could drop it from another session and the query running on the snapshot would immediately fail (rather than block me from dropping the DB snapshot).

I would recommend taking a closer look at the disk where the DB snapshot is created (since dropping the DB snapshot physically deletes the snapshot file created). There possibly could be speed issues there causing the delete of the snapshot file to simply take close to an hour...or there could be contention at the file level (something trying to back up the DB snapshot file while you're trying to drop the DB snapshot, virus tools scanning the DB snapshot file, etc.).