These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anti-Climax Boss: Once you knock out the Big Bad's fortress, you get a cutscene of your character walking up to him and shooting him in the head with a pistol without a word.
Game Breaker: The crossbow. Hit someone with it and the bolt explodes, killing them instantly. As the developers pointed out, the bolt travels slowly...but you can stop time. Oh, and it has a scope. It doesn't take that much patience to just use this to kill an entire base from long range.
Good Bad Bugs: In the unpatched console versions of the game, there's a bug that prevents the helmets worn by Krone soldiers from deflecting headshots. This lets you kill them with one headshot each, which is very useful since they can otherwise soak quite a bit of damage. This bug doesn't apply to the Krone police fought in the early levels, somehow.
Narm: Setting people on fire will occasionally result in them shouting "Arragh, my skin!"
Tainted by the Preview: One of the most devastating examples in video game history. While there's no doubt that the game would have bombed horribly had the final product been like the 2006 demo, some felt that version was at least trying to do something different.