Not just concepts, but play styles, as well. An all-martial party or low-/no- magic campaign, for instance, had always been problematic in D&D, and is once again so in 5e (less so, ironically, because of the 4e-isms of overnight healing, short rests, HD, feats like Ritual Caster, and the like). That's a style of play often attempted in past editions, but requiring extensive re-tooling to work at all (and I'd never seen it work /well/). In 4e, adopting such playstyles was virtually seamless.
The idea of a warrior-type - a 'martial' or 'non-caster' to put it generically or simply a typical fantasy-genre hero - being a natural leader has been with D&D for a long time. Fighters name level coming slightly earlier, at 9th, and bringing with it some low-level followers and the 'right' to claim territory and build a keep, in AD&D, 3e coming right out and saying that fighter tended to 'anchor the party' and to be party leaders (also parodied in OotS), the 3.x-adjacent Miniatures Handbook even had a highly-specialize, non-viable in typical D&D adventuring, 'Marshal' class (that was not an old-west lawman, but a European-idiom Field Officer commanding troops). Those ideas were there because they were common genre tropes, to the point of being cliched. The traditional hero was never a wizard, rarely had magical powers of any sort (might be under a curse or have a fated destiny or something), and typically either charismatic and a natural leader, or grew into leadership as part of his story arc. However, the game failed to put any of that into its mechanics. The AD&D Fighter never had a use for CHA, which was the only thing mechanic that impacted the morale and loyalty of followers, and never gained any ability to model tactical or military knowledge or talent, either. The 3e fighter, likewise, had no such abilities, no use for CHA, and too few skill points to even be good at jumping, climbing /and/ swimming (choose 2!) let alone a master tactician, had there even been such a skill in the PH, obscure attempts like the Marshal or various PrCs fared little better.
The Warlord made those sorts of concepts practical. Not only did it do that, it made them playable while side-stepping the issue of being 'party leader' - the "Leader" classes in 4e were support, they might be a 'spiritual leader' (Cleric) or 'battle leader' (Warlord) in a fluff sense, and thus help their allies with a Blessing or Encouraging Remark, but the player of a "Leader" role character didn't boos other players around. It also, seemingly unintentionally, opened up another kind of concept that had always fallen flat before: the contributing non-combatant or side-kick who doesn't seem to do much, but the party does better for their being around. Fans came up with a corner-case 'lazy' build that mostly granted actions to other characters, seeming almost like a bystander in the narrative. Folks had tried to do such characters in the past, the lower-level thief the party is keeping around for a specific task and needs to protect. It never worked well because there was no way for such a character to pull his own weight. With just the right choices you could make a 'Warlord' build fill that concept - it was a bit of a stretch, you'd have to end up with the odd maneuver where you had to actually take up a weapon and and least make the desultory attempt to hit something, but it was do-able. And it's not do-able now.
Hope that answered your question, Orlax.
