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.
YMMV: Mass Effect 1
Alas, Poor Villain: Benezia, Saren if you manage to convince him that he is being controlled by Sovereign.
Geth snipers can kill Shepard in one hit on most difficulties, and, if you don't take them out quick, they will. Geth ghosts can be even worse, as they hop around and never stay still and use the exact same weapon.
Krogan enemies in general.
Ear Worm: If you are planning to do side quests, this.
Genius Bonus: Binary Helix is working on a biological weapon for Saren based on a pesticide. Making it Saren Gas. Oy.
Good Bad Bug: There are also a couple of occasions to completely max out your Paragon/Renegade scores in a couple of minutes; however, doing so corrupts your save files, and makes your actual decision in that scenario discounted in the sequel.
That One Achievement: The achievements that require playing through the majority of the game with each individual squad member. This includes the sidequests and such.
Acid attacks from the thresher maws and rachni completely bypass your shields, no matter how strong they are.
Also, in this game only, melee attacks do the same thing, which means ME1 is the one and only game in the trilogy where ordinary Husks pose any threat.
The krogan battlemaster on Therum, at least if you go there early. Some classes may find the fight easier, however, such as the Soldier.
Matriarch Benezia, mostly because of the asari commandos and geth snipers that help her...and because this is often not only one of the hardest fights in the game, but also the most prone to glitches, making it a Luck-Based Mission for many (if Benezia decides to start throwing magically cover-ignoring Warps at you, you're done).
The Volcano Hunt mission in Pinnacle Station. You get more time on the clock for each enemy you kill, but the amount of time you get back is progressively less for each one. You will want to put your keyboard/controller through the screen after coming up 2 kills short of the top score for the 10th time. For some reason, the Volcano map is the only one with this problem - with the others, a sufficiently skilled player can keep the mission going indefinitely and have no problem racking up enough kills.
Pinnacle Station in general, really.
The rogue VI assignment on Luna. You have to clear three bases full of hovering drones that can kill you in one hit.
