Working off of my above post (now that I'm not writing it on a tiny phone screen), I can give a more precise accounting.
In terms of combat-related actions, you must have (or, for some of them,
really really should have, but don't
technically need) at least one of each of the following (one person may possess multiple elements):
- Weathering sizable or sustained damage without going splat; the "armored tank" as Gygax put it
- Focusing enemy attention where it is least harmful or most beneficial to the party
- Dealing good damage to several creatures at the same time
- Dealing damage from a distance
- Healing (though in general this is for "post-combat" rather than during)
- Dealing high spike damage to individual targets
- Closing the distance with the enemy, often to deny any terrain advantages
- Creating terrain advantages for allies or terrain disadvantages for enemies
- Providing force-multiplier effects (other than terrain), or removing sources that weaken allies
- Taking away enemy force-multiplier effects (other than terrain), or applying effects that weaken enemies
In 4e, these were organized into the four roles. Being a "Defender" class meant having baseline class features that supported 1, 2, and 7, and to a lesser extent 3, 8, and 10; it was almost always
possible to spend resources (usually feats and/or items, but sometimes class powers as well) so that any class could provide some or all of those benefits, but not guaranteed. A Wizard, for example, might not be very good at
personally taking hits, but if she were specialized in summoning or illusions, she could be creating extra targets on the battlefield that would cover 1 and 2 above without any extra costs to the party.
For 4e's other roles, Strikers usually hit most or all of 3, 4, 6, and 7, with some light splashes of 9; Leaders focused heavily on 5 and 9 but often picked up one or two of 1, 2, 3, 7, 8, and/or 10; and Controllers specialized in 3, 8, and 10, but often also offered 4 and 9 too. As you can see, there's a lot of overlap. E.g. 10 shows up in different ways under Defender, Leader, and Controller; 9 was a focus of Leaders but also appeared with Strikers and Controllers; 7 appears frequently for all roles except Controller, and that's only because Controllers usually focus on
areas and
multiple targets, while 7 is mostly a concern for melee-focused classes/builds. Etc.
We can do a similar analysis of social situations, though this is more loosey-goosey because (as has always been the case in D&D) there are fewer and lighter rules for social situations than for combat ones. Social interactions have both a broader scope and fewer defined elements, e.g. there is no equivalent of "HP" defined for social situations. (This would be part of why 4e proposed the Skill Challenge, even if the framework was very rough to begin with--with things left highly loosey-goosey, it's easy to fall into the trap of "mother-may-I" and harder to think critically about how one achieves success outside of "keep poking and prodding until the DM relents, rejects, or rolls.")
In social situations overall, e.g. covering as many
potential situations as possible, the group will need at least one person who can:
- Identify false statements made by others and the true motives of others
- Make convincing false statements and conceal the PCs' true motives
- Detect disguises and/or illusions and otherwise ascertain the true state of affairs
- Create convincing disguises and/or illusions and otherwise conceal the true state of affairs
- Speak eloquently about the party's intent, purpose, etc.
- Coerce others into doing as the party desires, even if they would rather not (whether by threats, offers, oaths/values, etc.)
- Stand up for the party's goals or desires in the face of persuasive force (e.g. resist the aforementioned tools of manipulation)
- Act as translator or interpreter for situations where language is a barrier
- Acquire and preserve important evidence and information for later use
- Persuade or manipulate large groups of people all at once (via performance, subterfuge, what-have-you)
It's possible there are more that I'm overlooking, but this is reasonably comprehensive. Most of these elements are covered in one of three ways: having a relevant background characteristic (whether a formal in-the-rules Background feature, or something separate from character features, e.g. a backstory element), having Proficiency/Expertise in a skill and/or a high ability modifier for the associated ability of that skill, or using magic. Unfortunately, as with the above set, some of these elements in 5e pretty much
require magic, whether as a hard requirement (you really can't make illusions unless you can cast spells) or a strong but still technically soft requirement (translation is
vastly easier with magic, preserving many things such as corpses is nigh-impossible without magic, and while it is technically possible to detect magical things without
detect magic, it's often damned hard.)
The other unfortunate side of this pillar is that it is often possible for
one single person to be legitimately excellent at
every single one of the above elements. The Bard class, for example, can theoretically fill every single one of those roles with high competence. A spellcasting-focused Lore Bard with high Cha and Wis can be borderline-unbeatable in nearly every category
simultaneously. Note that I do not say this because I think the Bard is a problem, because I don't think that--I think it's merely illustrative. I say it because it reflects how minimally-developed the mechanics of socialization are in 5e (and 3e; I do not have enough experience with 2e or earlier to speak about them), and that making them even
less developed does not (and I would argue
cannot) solve that problem. Some of it, of course, is that low-level magic is often an extreme binary of either fantastically useful or totally worthless (e.g.
disguise self is perfect so long as no one tries to touch you, and worthless if they do touch you;
friends is either utterly pointless because it makes people hostile a mere minute later, or fantastically OP because you don't care if the target becomes hostile). But some of it is also just that "have good numbers in 4-5 skills" is something most characters can achieve, and likewise "be able to cast some illusion, enchantment, divination, and/or transmutation spells" is something nearly every full caster can do by the time everyone has their subclass (and even many non-full-casters can do it).
Most people have an intuitive sense that if a single character could do excellent single-target and area-of-effect damage, buff others, debuff enemies, control the battlefield, heal, take big hits,
and work equally well at range or in melee, that that character would be overpowered and inappropriate. Even people who tend to treat "balance" as a stinky no-no word will speak out against something they perceive as capable of doing this. But as soon as you move away from combat, that intuitive sense disappears, and people seem perfectly content with having characters that can do literally everything in a pillar at high to maximum competence.