Unless all you play is COD, there are definitely other shooters that aren't on strict linear paths. The problem is in killzone it is no fun to explore. Walkable pathing is not clear. Objectives hard to see. Save system nonexistent and unclear, making venturing off the beaten path punishing. Shrub combat puts you at a disadvantage and not the enemies that see you just fine. Alarm system design makes this all that much harder, since its hard to see the alarm locations, AI pathing, and how to get them. "Manual" radar design that you can fail makes it more annoying to monitor your surroundings. Even the radar can be hard to see at times
I don't mind exploring and getting rewarded for it. But the could have tried to make that fun to do
There are some things there that I agree with, like the objective marker getting hard to see due to the screen being busy (Forest level mostly), but we're talking about 2-3 seconds to notice it. The game also seems to have a bug in later levels where the marker doesn't appear at all.
The alarms were also a pain to find, so they could've done a better job with that too. And last, but not least the saving system which pretty much sucks ass.
Everything else I really didn't have an issue with. Never did I have an issue with the "walkable pathing", nor with the combat (thought the combat was as diverse as I've experience it in all KZ games, thanks to the OWL), or with the AI paths. I just moved around, ran, jumped, slid and used the OWL to my advantage. Manual radar also worked well for me, although I admit that the game was practically built around it. Specially the forest level.