I should presage this by saying that the DM in question was a pretty good one other than my complaint here, and it was his first time DMing. I should also say that he was a really nice guy feeling his way into running his first RPG. His DMing was certainly streets ahead of my first attempts (and most other first-time DMs I've seen). However, here's my moan...
So the DM in question ruled that natural 1s and other natural low rolls (no matter the positive modifiers) were critical failures. I think he saw this kind of thing on a Let's Play series, and perhaps over-applied it, I don't know, I don't watch them. He applied this ruling not just to hit rolls, but all skill checks. The kind of outcome might be the attack killing a friendly NPC, or a failed attempt to throw a grappling hook meaning that the PC drops and loses the entire rope or falls and gets pulled in.
It was funny the first few times it happened, but it soon gave the impression that our party was a bunch of incompetent idiots, as any encounter would lead to at least one or two pratfalls. It actually had a noticeable chilling effect on players trying anything remotely interesting. I recall one player forgoing a chance to attack on his turn because he wouldn't dare traverse a low bench as he might fail the inevitable Acrobatics roll and then end up both taking damage and ending up prone. In my view, players should be encouraged to do heroic things, not be terrified to do the most mundane things. One fellow player quipped to me that he was scared to open a water bottle in case his character fumbled the roll and died of a cut.
Now, I realise that D&D combat isn't realistic, but my parallel experience with real-life sword fighting just rubbed salt in the wound. At this time I was doing historical sword fighting. I was very, very bad at it - embarrassingly so. However, in dozens if not hundreds, of bouts, I never once accidentally dropped my sword or fell over. And in the hundreds of other bouts I saw, I rarely saw an accidental sword drop and never saw a fall (throws and disarms yes, pratfalls no), and I never saw bystanders be hit. And most of us were beginners. D&D combat shouldn't be less cinematic than real-life combat done by rank amateurs.
The DM's style was otherwise pretty good, but this aspect was very annoying, and actually was a significant factor in me leaving (there were other factors which weren't his fault).