Li Shenron
Legend
My chief concern is of how well the system will work with my campaign world. The campaign is intended to be low magic, a setting in which arcane casters and magic items are a rarity. Divine casters are somewhat less rare, but still a very uncommon occurrence. How well would 5E support such a world? As I understand it, 5th edition characters are a lot less reliant on magic items to maintain an appropriate power level (correct me if I'm wrong), so I'm not too concerned on that front. However, I have seen a lot of character examples in which classes that I would not normally associate with magic by default have some level of casting abilities. I understand that there are a lot of options with how characters are built, and taking different paths will give different abilities of different natures, but in not knowing the specifics I don't have a grasp of how prevalent these magical paths are. Would it be possible or even reasonable in 5th to have a party of six players with only one character among them who has access to arcane spells? And one with full access to divine, or maybe two with limited divine ability? Can rogues, fighters, etc. be played completely without any magical ability? Would placing such restrictions on a group severely limit character options, or does 5th give enough non-magical choices for characters to still offer some variety? This alone is probably the issue that will determine whether I convert over to using 5E.
You have separate issues here: magic items, and spellcasters.
First of all, 5e should work also in complete absence of magic items, so that's settled.
As for spellcasters, you need to be very sure what do you mean by "low-magic". You mention that you want spellcasters to be rare, not necessarily you want their power to be low, which is an independent issue. This is good, because it means you don't need to do any change to the existing classes or other material.
Now that we've narrowed the problem down to mere rarity of spellcasters, consider also the separation between PC spellcasters and NPC spellcasters... Because you can totally have a party of ALL spellcasters, and then have ZERO spellcasting NPCs ever, and you'll still have a world where spellcasters are nearly non-existant, the only ones just happen to be concentrated in the player's group

Personally, if I want to set a campaign's theme to "low-magic" in terms of spellcasters rarity, I wouldn't necessarily restrict the races and classes available to the PC, but I strive to make the PCs feel truly alone and different from the rest of the world.
Among other things this could mean to assume:
- they'll never find another of their kind
- they'll never find scrolls or spellbooks to copy spells from
- they'll never find magical training or knowledge through RP
- the rest of the world will strongly react to them (fear, awe, hatred, anything...)
Still, it's for the best to discuss with your players, because the final thematic result will be very different from having truly no spellcaster in the party. Ask them which "theme" would they prefer from the game, between being unique marvels in a near-magic-free world, or magic-free heros in a near-magic-free world.
From the point of view of capabilities (i.e. lacking spells to save the day) I wouldn't worry at all. The game will work just fine, and if anything, it might actually force the players to think more than usual about how to solve certain challenges that can be traditionally bypassed with spells.
While not nearly as pressing of an issue, I did have another concern with one 5th Edition mechanic I had been reading about: the Advantage/Disadvantage system. I don't think I've read a single bad thing about this system, and from what I've seen it seems to be universally liked as a useful simplification over previous systems (which is a good thing). I understand the basic mechanics of it (roll 2d20, use higher or lower depending on situation), but I admit I don't have a good knowledge of what all determines advantage or disadvantage. That may color my perception, but as I see it the game loses something in this mechanic.
...
Is the Advantage/Disadvantage system a part of the rules that can be omitted in favor of something else? Or can anyone who has played the system offer some encouragement or clarification as to why it may not be as bad as I am expecting?
My suggestion is not to worry about when it's appropriate to grant (dis)advantage, just grant it ONLY when the written books tell you so.
Thus when a PC has a spell/ability that says "grants (dis)advantage to someone", then do it, but don't do it by your own volition in any other case. Don't even worry much about whether some conditions or environmental circumstances grant (dis)advantage: if you happen to remember that, then yes proceed as by the book, but if you forget, it's not a big deal.
I think (dis)advantage has some built-in flexibility, but mostly only in the direction of making it more common by granting the benefits to reward a player's creativity or tactical thinking, but it also carries the danger of going very easily overboard.