What if a client grabs a lock, goes "out to lunch" holding the lock but not making further queries for a while -- and during this time the client abruptly dies or the network connection is severed? [In the case in question this scenario is somewhat common and critically important, not something that can just be shrugged off.]

Some admittedly brief googling suggests that SQL Server won't notice this. Microsoft's closest article is one on finding and fixing this issue manually as an administrator and there is an old article explicitly stating this as a gap in SQL Server as compared to Oracle.