Allocating each Job to one Processor without Sharing the CPU Time is Single Threading.
Some processes, such as the merge phase of a sort/merge (sorting can be subdivided...) MUST be run single thread. Other operations (balancing b-trees, etc) are single thread and tend to lock out other accesses. Therefore, single thread performance is critical to reasonable operations against a DataBase (especially when adding new rows).
maximum I/O connectivity
Ability to connect more Input output devices.
Mainframes excel at providing a convenient paradigm for HUGE disk farms; While SAN devices kind of weaken this to some degree, SAN devices mimic the model of the Mainframe in connectivity "tricks" (at least internally).