Why do pluses on vanilla weapons and armour cap at +3, as opposed to +5 in previous editions? Is there meant to be a different style of item-based progression in DND5E, or am I just missing something here?
D&D 5th edition was designed with something its designers call "bounded accuracy." A full description and analysis would require more text than is appropriate for this format, so I recommend a search of the topic on the wider internet.
To very briefly summarize bounded accuracy, all modifiers (and also target numbers, such as AC) in 5e are limited to a range that is much smaller than previous editions. One of the main results of this is that a large number of very easy monsters, such as goblins, are still a threat to a party of high-level adventurers. This is different from the last few editions, where a single high-level character could defeat an almost infinite amount of goblins.
Too large of bonuses from magic items would break this desired mechanical behavior, while smaller bonuses are more significant than in previous editions.
Bounded Accuracy is a key element of 5th Edition's design. In order to keep the lower level challenges relevant at higher levels, they've tightened the range of bonuses. This keeps a flatter curve and allows challenges to be relevant at a variety of levels.
For example, at low levels, a single Kobold or Goblin is dangerous to a single adventurer. As the adventurer advances, he can take on larger number of these creatures without fear of imminent death (his HP are higher, his hit chance is 10-15% better, etc). They still have the possibility of taking the adventurer down with overwhelming numbers, but the adventurer ceases to be concerned with a one-on-one encounter.
If you compare this to 3.5, you can see the whole progression is compressed. Instead of "base attack bonus" (for fighter types) going from 1 to 20, it now goes from 2 to 6. A +5 weapon in 3.5 is a 25% improvement at the top of the range. In 5E, a +3 weapon is actually 50% on the range - a much more potent increase. (Obviously stats factor in, too, but for the direct comparison here, I'm ignoring them.)
In prior editions, there was no actual cap; some weapons above even +5 existed in canonical materials. Modifiers stacked, prior to 3E, all of them combined.
In the development process for 5E, the designers announced bounded accuracy.
Bounded Accuracy incorporates several other limits:
- Attributes for PC's in the range 3–20
- Modifiers from attributes from –4 to +5
- Tool modifiers capped at advantage, no direct DM's
- One spell's modifier (use the best), typically under 6 (bless is a d4)
- One proficiency Modifier; 0 to 6.
- One weapon's or tools magic modifier of +0 to +3
This gives a range of –4 to +19 for die rolls, with non-magical non-attacks reasonably in the –4 to +11 range.
This was an attempt to reduce the math to reasonable ranges. Largely, it succeeded. This allows for individual modifiers to be more important, and for given threat creatures to be useful for longer in a campaign. Bounded Accuracy means a minor monster with a +5 attack (pretty typical CR ≤1 ) can still hit even plate+3 (21) better than nat-20, and Plate3 with Shield+3 is still reasonably threatened by lower CR monsters in the +6 to +8 total attack modifier.
Further, reducing the ranges makes each increase feel more important.
Further still, most of the monsters which required "+1 or better to hit" now are hit by magic weapons, even +0. This makes lower ratings more useful.
These changes and others make the expectation of having magic weapons, present in 3.X and 4.X (and to a less explicit but stronger extent in AD&D 1E and 2E), no longer a thing. But it starts by not making target numbers of more than 30 common... and Shield spell plus peak magical armor in the 31 range is still hittable, while a peak to hit is in the range of Nat 20 + 16 for 36.
Bounded accuracy. Proficiency ranges from +2 to +6, compared to previous editions having a constantly increasing BAB or constantly decreasing THAC0 (which is mathematically equivalent to getting a large + to hit). This means that relative to the rest of your bonus, +3 is huge. It's equivalent to hitting like a character 12 levels higher than you.
Due to Bounded Accuracy. If you check the monster manual, you will see that the AC cap is much lower now than it was in other editions. Same for Attack bonuses (that cap at a +11). Now a +3 is effectively making your character from 30% to 50% more able to hit a creature, so it's a much bigger deal.