There are two main facets that make a game’s Lord have good game-feel.
First, and most obviously, the unit offers good combat or utility. They hold their own, you’re not babying them every turn to make sure they don’t die, they’re actively contributing to your army.
Second, and this is where canonical FE struggles a bit more, the Lord has to be able to share the stage. FE1/3/6/11/12 are almost exclusively maps with an objective to capture, and because of this, Marth/Roy have this “Ferry to throne” feel for players trying to test their skills or otherwise. In turn, this makes them feel worse as units because you aren’t using them as units that much.
And of course, a Lord can’t be a Lord unless they’re a central figure to the story and the story, in some manner, cannot be itself without them. If the plot could go on without the Lord, the Lord has failed to be sufficiently part of the story for it to be a good example.
I would argue - rather extensively - that Lords do not have to have
Class: Lord, they definitely don’t have to
Uses: Swords, they don’t have to have approximately 18 HP, 5 Strength, 5 Skill, 7 Speed, and 5 Defense at level 1 (Marth, Roy, Eliwood, and Ike).
Lords don’t need to have good stats or growths and can be extremely unconventional, as long as it doesn’t feel like an active drag to use them. They have to be a likeable person or at least not easy to hate, because the player should want to see them succeed because it is that success that the story is going to most likely build towards.
Lords don’t need much to be good, but to be great, I believe they need to be in a well-written story that lets them show their character’s depth and breadth off. They need to have a varied cast of friends to ping-pong with, to build the world and to showcase their own beliefs about that world. They need to have gameplay use that isn’t “Is the way to end the map”, they need to have something unique to themselves.
Really, all of these same elements apply to any character or unit. The thing that makes the Lord be the Lord is that the story is about them or requires their participation.
A great lord is, in essence, just a greatly written character that is at the story’s center, and as a unit is competent and consistent.