Difficult. One of my players told me that yesterday.
I think the expertise system should have been used more. Every background should have had some kind of expertise tacked on.
Actually, at a certain point in the playtest, knowledge was represented by a +10 bonus. I think that actually is the kind of bonus you need to make a knowlwdge skill worthwhile.
And now I just adjust the DC depending on the person's background or proficiencies.
Yup... there are a lot of ways to do it. I've been considering several different options for my next campaign for many of the same reasons that have been brought up.
Right now the biggest thing players and DMs have to wrap their minds around is that in 5E skills
are not masterful knowledge. They are merely wide swathes of ability that they were born with or have worked on over time. Nothing major or having to do with long-term study.
Some people are just better at hearing music and being able to carry a tune. They are the ones who are Proficient in Performance. They aren't Masters... they are just proficient, Some people have read more books about magic. They're the ones who are proficient in Arcana. They don't know everything, but odds are slightly better that they will know something that another doesn't. Some people aren't very strong, but they have at least learned the techniques of proper strokes for swimming and proper hand and foot placement for climbing. Thus they are proficient in Athletics. They aren't world-class athletes, they're just competent. And so on and so forth.
However, if you actually want to get across
masterful knowledge, study, and ability... those don't come from the Skill system... they come from all the other methods the game uses to exemplify it. The "double proficiency bonus" method is one way-- given to all dwarves via Stonecunning and given to rogues and bards via Expertise. There's also the Feature ability for each Background, which gives a person one thing that they just know or can do-- and for which they never have to roll for anything, the game just gives it to them. There's the specialty feats like Actor, Athlete, Linguist, and Observant, where they specifically call a person out as being a master at something specific, giving the person some specific special abilities while also leaving it up to the DM to adjudicate what else "story-wise" the character might get. Then of course there are all the Inspiration characteristics (BIFTs) where a PC gets to specifically call out things about themselves that they are, do or know and for which they get Inspiration (advantage) for.
This is all much different than 3E, which pretty much used the skill system as their way of making experts expert and the untrained virtually useless by comparison. So for 5E, you really need to take all of these different methods the game offers up and decide which ones you like the best and then really play it up. Raising DCs for the non-proficient is one way to do it. Making Backgrounds more concrete knowledge wherein if something falls within the background's purview the PC doesn't even have to roll, is another. Making the Inspiration characteristics (BIFTs) less like the ones in the book and more like FATE Aspects... wherein you can specifically call yourself an "Expert Negotiator" or "Silent As The Wind" and thus gain Advantage on rolls that pertain to them, is another.
I myself have considered the idea of "Mastery" wherein a PC that has already proficiency in a particular skill can obtain the "double proficiency bonus" of a small slice of that skill (a la the Stonecunning dwarf feature which gives double proficiency to Intelligence (History) checks specifically with regards to the small slice of stonework and structures). I'm thinking for myself perhaps that PCs have one Mastery per character level, and whenever they roll a 20 on a skill for which they are already proficient, whatever that very subject matter was about can become one of the character's Mastery subjects if they so choose (showing off the very specific section of ability or information for which they are much more advanced than other people.) So for instance, if a PC who has proficiency in Religion rolls a 20 on a monster knowledge check about the undead, the player could decide that "Anatomy of the Undead" is their Mastery subject. And from then on, whenever I was to call for an Intelligence (Religion) check having to do with the undead, they would get to use double their proficiency bonus. Or perhaps a PC is making a Dexterity (Acrobatics) check to cross the peak of a roof and rolls a Natural 20... that player could decide to make "The Balance of a Cat" a Mastery ability, and will get to roll with double proficiency bonus for any future Dexterity (Acrobatics) checks to do with balancing.
Does this slightly step on the toes of the Rogue and Bard's "Expertise"? Eh, maybe a little. But then again, Expertise is double prof bonus for the
entirety of a skill, not just the small sub-section of knowledge Mastery would give. Thus, Expertise is more universally useful. Plus, of course, Rogues and Bards could also pick up Mastery on top of what they already have Expertise in, so they'd know even more.
This idea solves a lot of things for me. One, it makes rolling a 20 on a particular skill check occasionally have even greater import, as it can become the indicator of a PC's super-knowledge of a particular subject. Two, it makes more use of a mechanic that I think is used to great effect in the Dwarf, and which I think should be used more often across the game. And three, it leaves the Advantage mechanic available for other times where its use would make a lot of sense (on top of the other skills/features/bonuses a PC already has.)
I'm looking forward to seeing how it works in play.