Snow Leopard includes a host of new features with performance in mind, such as Grand Central Dispatch (which allows Snow Leopard to better take advantage of multi-core processors) and (which lets Snow Leopard use the graphics card to do general computing tasks). This has uses beyond science, 3D, gaming, and such, but could be put to work for everyday tasks. Also, Snow Leopard should use much less disk space; Apple claims you could regain over 6 GB of disk space by installing Snow Leopard, but your mileage will most certainly vary.
As a point of comparison, Windows 7 Release Candidate saw a slight performance boost over Windows Vista in our .
Snow Leopard will be a $29 upgrade for users of Leopard ($49 for the five-license family pack). With previous upgrades, you would have to pay the full price -- $129 -- even if you were upgrading from 10.4 to 10.5, for example. Apple probably reasons that most users won't be able to see the new features and changes, which Apple has traditionally used to sell Mac OS X upgrades, so Apple will cut Leopard users a break this time. Even during the keynote, Apple positioned Snow Leopard as "Leopard: Second Edition" instead of a brand new version. It seems to fit in between a service pack and a full-blown upgrade, so it's priced appropriately.