The original Western Digital Scorpio Blue 1 TB was 12.5 mm thick, which was once the standard thickness of 2.5-inch drives. With the advent of thinner and thinner laptops, that size often doesn't fit any longer. The new Scorpio Blue 1TB measures 9.5 mm, which does make it suitable for most current laptops. It's true that many models are now even 7 mm thick, but as of yet that is still a minority.
If you wonder how Western Digital accomplished this, they did it by completely overhauling the interior. The original Scorpio Blue 1TB had three internal platters, and the new one has only two, with 500 GB each. That also means that it has a higher data density. 500 GB is currently the highest capacity available for 2.5-inch platters.
We will see later on that the higher data-density makes the Scorpio Blue 1TB quite a bit faster than its predecessor. It's still a 5400 RPM drive, and for real performance you still need a 7200 RPM drive, and these don't exist yet. Ok, there's one, the 1 TB Seagate Constellation.2 2.5" 7200RPM disk, but it's meant purely for servers.
The new 9.5 mm Scorpio Blue 1TB costs an average of £119, the old 12.5 mm one £97. The price per gigabyte is not bad at all.