On Left 4 Dead 2

Left 4 Dead 2 is a much more rounded, enjoyable, and downright fun offering than the original. There are several reasons why:

1. The Music
Reprising the main themes from L4D, but altering them to the flavour of each campaign gives the individual campaigns a unique identity, but retaining the core feel of the original game. The result is that each campaign is now more memorable and has a great sense of 'place'. Whereas L4D was your typical Dawn of the Dead affair, L4D2 creates a much deeper and more original character to its locales, not only making it stand apart from other video games, but from Zombie fiction in all media.
Also, the 'seams' between the different pieces of incidental music have been more effectively covered up this time around - the music no longer cuts from 'spooky' to 'frantic' and back again - it has far subtler gears in between; you can be clearing an area of common or garden infected, and when a few more turn up, or maybe a charger crashes the party and you start to feel things are turning against you and you've lost control - the music has already shifted through cues to reflect that mood without you even noticing, though now you're panicking, and there are frantic trumpet stabs and pounding drums to reflect that, and heighten the tension.
Also, there are more unique cues linked to locations and landmarks, which really helps the level design to stick in your memory - seeing a silhouetted ferris wheel behind a sign for Whispering Oaks Amusement Park, and hearing a gothic, jaunty circus theme near the beginning of Dark Carnival is a great example of the for-shadowing that Valve really excels at.

More later...