+ Infinity. There have been so many threads of people asking clarifying questions about the rules or otherwise not understanding game mechanics. I can't imagine how many other confused people there have been who didn't end up posting anything here.
The game should contain all the information necessary to understand it. I don't know why it doesn't. Players should not be required to exit the game and google "D&D 5e rules" in order to understand how to play.
1) Tooltips - the game needs a system that a) actually shows what kind of modifiers players are getting and b) has tooltips for those modifiers that explain what they are. This is implemented in some systems, but hasn't been implemented in some areas where those modifiers can get a bit convoluted and confusing.
2) An option to flip open the BG3 version of the Player's Handbook and see why things happen the way they happen. Basically, give us an in-game encyclopedia. Sure, a lot of players don't use in-game encyclopedias (and they're the ones who will need tooltips), but some players would definitely benefit from having a feature like this.
Yes, with the addition: various terms we encounter while playing (Advantage, Saving Throw, Tiefling, Backstab, Disengage, Nature Check, Baldur's Gate, etc) should be highlighted. When hovered over or clicked, they should either bring up a pop-up explaining them or open the encyclopedia to the relevant page.
Also, the game obviously needs a better tutorial. Maybe a more laid back one, where you're not trying to escape a burning, flying, devil- and mindflayer-infested ship that is in hell and so can focus more on learning the mechanics instead of the spectacle.