This is what's known as "damning with faint praise"

Well I havent played it so I can't comment yet, but it sure looks great.

No love for Killzone Mercenary

It was one of key points in Killzone.

IMO, you'd not notice it if KZ and KZ2 is all you've played... it's there.. but it kinda isn't. Most characters are also rather annoying than helpful to the story in those.

KZ3 and on, no idea.

Well, he would also say it if he would believe that it will be the worst...

But I have to agree, was never a Killzone fan but the next one looks interesting.

It would be shocking if he said it wasnt. ^^

The overarching story is good, though the games themselves haven't really touched on that depth very well.

If you take the Helghast at face value, for example, you might just see a facist, militaristic people who fit the role as villians to perfection.  The ISA, the "good guys" in the Killzone games, seem like the heroic force attempting to protect humanity.  The reality is that both sides acted with greed and power as prime motivations.  The Helghast tried to secede from Earth's control and were crushed and exiled to Helghan.  Earth wasn't going to give up on the wealth generated by Helghast assets.  They also used the situation as an example to other colonies.  This resulted in the Helghast becoming as you see them now, seemingly driven by hate and revenge.

This, I think, fits in well with a lot of human history.  The winners become the good guys, the losers are the "guerrillas" forces causing all the problems.  Think about the role the US has had in other countries over the last fifty years, or the British Empire.  What if the British had won the War of Indepenence in North America?  History would have a different perspective.

With Shadowfall, GG is obviously trying to give us a better taste of that.  I think they've read all the fan feedback that asked for more depth.  We've seen hints so far that all is not black or white, good or bad in the game.  The whole structure is even a nod to the Berlin Wall and the Cold War from the end of last century.

I think this trailer is telling us that things won't be as simple as in past games.  That's what I hope, anyway.

Looks like it is not just the best KZ, but the gameplay simply looks amazing. Best launch title by far imo. It will be one of the best shooters for awhile.