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.
Alternate Character Interpretation: Is Gordon just an innocent scientist caught up in a much larger plot and forced to take extreme measures to survive, or did he deliberately cause the Cascade and is trying to escape and cover his tracks?
Disappointing Last Level: Xen. Previously known as Xen Syndrome, Half-Life was the founder of this trope due to its annoyingly-designed jumping puzzles seen throughout the alien world levels.
Fridge Logic: In Opposing Force, how exactly is a Marine recruit a corporal?
Genre Turning Point: For the First-Person Shooter genre. Prior to Half-Life, First-Person Shooters were generally "shoot everything you see" with no sense of tactics or strategy, and the extent of puzzles were just simple "search for the key and lock" puzzles. Half-Life changed that with more complex enemy A.I., requiring much more care on how you progress.
Near the end of Surface Tension, you reach a map layout that requires you to find, and escort, a security guard to open a door to the next area. Getting stuck in doors usually does a slight bit of damage to you. However, this door does the exact opposite and HEALS you instead; to the point that purposefully getting stuck in the door will allow the player to obtain health levels beyond the 100 health limit. Want 4000+ health? Just get yourself stuck in the doorway.
Due to the game using scripting quite often, scripted behaviour can be heavily abused. Especially useful with one Gargantua - it's spawned neutral and if you remove trigger of chasing script (what can be done with pistol, FYI), it will remain as so. This can be done twice in whole game. Miniboss fight easily averted.
Tricks that allow the player to skip entire sections of the game.
Near the end of Office Complex, you can push a security guard towards a locked exit door to open it from the locked side, allowing the player to skip having to traverse through the freezer portion of the level.
At the very beginning of We've Got Hostiles!, you can glitch the scientist to open the security door which will allow you to click the button to open the silo door to the Blast Pit; effectively skipping the entire chapter.
Seinfeld Is Unfunny: The vast majority of the game's innovations have since been copied by every single first person shooter to come since.
Ugly Cute: The Spore Launcher from Opposing Force is a repulsive alien creature that Shephard can use as a weapon. But if you hold still for a bit, Shephard will pet it and for a moment itís just adorable.