Killzone 1 isnt a good shooter.
The aiming feels awkward and imprecise (i.e. the sniper rifle), the framerate has got to be in the low 20s the entire game, the encounter design stinks, the AI is wildly inconsistent (it's easy to tell that KZ1's AI is complex, yet poorly implemented) and there isnt much enemy variety. Also, the Helghast take at least a half of a clip or more to kill.
However, KZ1 did have some cool aesthetics that werent all that common on the PS2. The reloading animations were quite detailed (and lengthy), and most notably there was a blurring effect that was put to good use--as you exited the sprint option, your rifle blurred back into focus in a cool manner--the art direction was quite good, and some aspects of the presentation--score,elements of battle chatter--were done fairly well.
KZ2's framerate looks like its above 30 (save for a few instances), overall the Helghast seem a lot smarter this time around, and it's clear that the game's cool aesthetics have not been implemented at the cost of the framerate.
I cant speak on KZ2's encounter design (although it does seem pretty freaking intense), but I know there will be much more enemey variety this time, because Stuart Tilley, Guerilla's SCEE producer, has stated that there are about 10-12 different Helghast classes-- that each have their own behaviour-- all of whom are designed to keep the player on his feet via different methods of attack.
I've read different reports on the "feel" of KZ2's weapons and first person cover system, and most of the feedback suggests that while it does take some getting used to, it doesnt take away from the immersion factor.
I think KZ2 will be a fine game. I dont know if it will be an instant classic though,that depends on the AI and how well Guerilla uses those weather weapons. I can imagine some pretty wicked encounters if weather is put to good use. There are hints that Guerilla is doing just that--one of the KZ2 multiplayer levels seems to be subsituting rain and snow for falling ashes.