I wrote a way back about a hidden problem in 5e with advantage. I've recently become aware of another, perhaps more insidious problem: the flavor/fluff writing and the mechanics *do not match*.
Basically, most spells or class abilities have the format of a title, a sentence or two of fluff/flavor, then a mechanical description. But HOW OFTEN does the first sentence not really match the mechanics?!? Let me give an example that happened in play very recently:
So we had an encounter with a weird monster (I'm GMing gates of firestorm peak, an excellent 2e adventure I converted to 5e.) and the party paladin decided to "detect evil, paladins can do that in 5e right?"
So here is the power. I have put the first "fluff" sentence in italic
The presence of strong evil registers on your senses like a noxious odor, and powerful good rings like heavenly music in your ears. As an action, you can open your awareness to detect such forces. Until the end of your next turn, you know the location of any celestial, fiend, or undead within 60 feet of you that is not behind total cover. You know the type (celestial, fiend, or undead) of any being whose presence you sense, but not its identity (the vampire Count Strahd von Zarovich, for instance). Within the same radius, you also detect the presence of any place or object that has been consecrated or desecrated, as with the hallow spell.
The player tried to argue that the effect would detect evil... but it doesn't! It detects fiends, celestials and undead (and consecrated/desecrated stuff). The thing was plenty evil, but it was technically speaking an aberration but it was such a vile one (and kind of undead-ish) that I ruled it as detecting evil, but RAW (and probably RAI) it shouldn't have.
In my experience, a lot of casual players are... not super aware what their powers/spells do. So they look at the name of a power, read the first line, think "that looks cool and makes sense for this situation, let's do that!" This is how they learn the game - by playing, by trying things and see how they work "live". And all these powers/spells that have a miss-match trip up these casual or new players, and there is no need for it. There is no reason why the first sentence couldn't have said "The presence of extraplanar beings and undead register on your... "
Here is another example from a few months ago, in a Drakkenheim game.
A fighter was surrounded by 2-3 foes. The party druid decided to help their fellow PC out by casting a spell at one of the bad guys. She looked at her selection, and chose Ice Knife. She cast it at the foe and... it turns out that the ice knife is an *ice grenade*, and the spell hurt everyone in the area, including her fellow PC! Let's look at the spell and what happened:
You create a shard of ice and fling it at one creature within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 piercing damage. Hit or miss, the shard then explodes. The target and each creature within 5 feet of it must succeed on a Dexterity saving throw or take 2d6 cold damage.
So once again, the casual player takes a brief looks, thinks "this looks good" - and it turns out the ice knife, which sounds like a single target spell, is actually an area of effect spell. It's a good spell! But it's not what the label says, and a more "serious" player would not have made that mistake... but neither would have a casual player if the spell had been named properly.
These experiences make the game less fun and harder to learn, and could be fixed easily with better writing.
There are several more examples of this if you look for them. The cantrip friend, for example.