@ selnor
The trouble is KZ 1 was really poor.
That appears to be an exaggeration, the game was not poor, but according to most reviews suffered from technical issues. The guys at 1up stated they believe the game was too ambitious for the PS2 to handle and everything significantly wrong with the original Killzone has been fixed in Killzone 2.
Other than those needed improvements Killzone 2 is a much bigger project and is being polished to extremes. Most people who played it were mind blown, multi-player is said to be very fun, single player campaigns extremely solid, etc.