Hmm...
Killzone 1 was an average shooter at best but was more grounded and had a btter player character than it's sequels (Jan Templar).
Killzone 2's Single Player was lacking but I wouldn't call it mediocre. It was (and still is) arguably the best looking console FPS with interesting locales, great A.I. and 2 of the best protaganists of the current gen in Visari and Radec. This is offset against poor characterisation, and a poorly constructed story that fails too take off with no explanation behind the history of the War unless of course you've read the great backstory hidden away and now removed from the Killzone site. Looking back on it the best 3 characters in the game
But the Multiplayer! IMO one of the best online experiences of the generation. Warzone with 32 players, Spawn grenades (When used properly), some pretty good map design and well balanced classes made for an experience that I personally found more enticing than CoD, Halo and BF (console versions). Oh and let's not forget about the trademark 'chirp'.
Killzone 3. Singleplayer was a marked improvement yet still has the same problems i.e. Uninspired and forgettable characters along with a meaningless story that again fails to draw from the amazing history and backstory that they created for the universe. This time the graphics are a bit more smooth in terms of aliasing (MLAA) and it containes a greater colour pallete etc. but I feel that this hurt the games art direction and lost the whole gritty feel that they went with in KZ2. Protaginists are laughably bad and I didn't find them interesting at all. The magic that was the 'Star Wars' inspired KZ2 protaginists couldn't be matched. The infighting amonst the ranks of the helghast was interesting but again the characters weren't. Mech fights, tanks etc were all interesting but served no purpose
I'm not even gonna mention the Multiplayer other than the fact that it was a massive let down apart from the new game mode.
Killzone Liberation was a brilliant portable game but as has been said it is oftne forgotten due to the fact that it was a PSP game and also because it was a top down shooter.
Realistically both upcoming games look really solid but we still haven't seen anything to suggest better characterisation and an improved campaign in terms of story and presentation. Improve on these along with better pacing for Shadow Fall and bring back The core of what made KZ2's MP so good. Forget about what CoD and others are doing and become unique again.
Mediocre? I think not, but destined for greatness? We'll see.