Absurdly useful UX expansion that I just wish more mods would support

Item Descriptions quickly became a mod I just do not play without anymore as it's so lightweight but does so much.
As someone who started playing all the way back in 2011, I am intimately familiar with every aspect of old Minecraft, but I often forget about so many new features they added to the game since ~1.2.1 or so. Item Descriptions genuinely helps a lot learning about all the new thingamajigs and recalling what stuff does.

It also just makes the UI feel a little bit more "complete" and I can imagine it being very, very useful for new players to the game.

The descriptions do not get in your way because they're toggleable, they are customisable and have mod support out-of-the-box simply using regular locale files in resource packs, and they integrate fabulously well with other mods like Spyglass Improvements or Field Guide.

I recommend also checking out the Mod Descriptions resource pack that adds a plethora of descriptions for all kinds of mods. "Hey Wiki", the mod that allows you to open the Minecraft wiki article on items using a hotkey is a great companion, too.

The only 'downside' to Item Descriptions is unfortunately something that isn't their fault at all: namely that many many mods do not support it and that even the aforementioned Mod Descriptions addon – while extensive – is still missing a ton of popular mods. I really wish more mod authors would add Item Descriptions support. A lot of mods do add descriptions, but they all reinvent the wheel and do their own thing and do not integrate with Item Descriptions even though it's right there.