Greetings, I wanted to start a positive series where we talk about various D&D concepts and what makes them cool and appealing to us as players, gamemasters, and lore readers.

I'm going to kick it off with the Elemental Planes and their inhabitants, an aspect of the setting I personally haven't been interested in. I don't have anything against them but they haven't been a major part of any campaign ideas or headcanons I've come up with. My speculation for their appeal is this: the Inner Planes provide an environment that is alien but comprehensible. Air, fire, water, and ground/rocks are all things we see in daily life and now facts of scientifically, so there are informed decisions that can be made when imagining worlds where one has primacy over the others, not just guesswork. The same could be said of their inhabitants, though as a fantasy game we're not bound to strictly realistic characteristics (see the salamanders which are based off antient folklore). And because there's nothing enforcing them to all be on one side, there's room for multiple nations and factions within the Inner Planes.

That's my piece, but I want to hear from more committed Elemental fans. What do you like about them?

there are informed decisions that can be made when imagining worlds where one has primacy over the others, not just guesswork.

This is the part I like, but on the creature scale. In my campaign I've added four second-tier elements that are blends of the main four and they all have a rock-paper-scissors type relationship that the players can leverage in fun ways. Elementals are the easiest and most straightforward type of enemies you can use in this way, although I've kind of overlaid them onto demons so every demon has a specific elemental affinity and the associated strengths and weaknesses.

As written I think they're kind of cool as the forces of nature personified. I'm not big into the Inner Planes lore and usually toss that bit out, but trying to characterize fire or wind as living creatures is fun. What would they want an how would they go about getting it?

Good topic.

For me, Elementals and their plane folk, and the planes they come from are fun because, at least as I play them, they're dynamic.  I think my Outer planes are sort of "entrenched" alignment/idealogically, almost to a fault in my own game cosmology.  The nature of the elements for me is mutable temperament.  In really broad strokes: Water can flood catastrophically but it's also a life bringer to so many.  Air can be tempestuous but also tranquility.  Fire is incredibly destructive but can also illuminate and provide comfort through warmth (and to put a darker tone on it sometimes cauterization is a life saver).  Earth most folks probably think as the dullest or most static, but at least in the IRL we live in a world that's formed by "tension" between tectonic plates, in many fantasy worlds one could say the earth elemental aspects are the part that either holds the worlds together or sunders it, so to speak.  

I really like the mixed spaces where elemental planes meet, I really like Mephits in capacities ranging from utilitarian/instrumental aids to just nuisances.

For me the elemental worlds/planes, characters just don't know what they're going to get.  The elements ends and agendas are just different from the alignment agendas influencing the prime material ... something that was a bit more apparent in prior editions where Druids were, I feel, more elementally aligned (the whole true neutral is more "beyond good and evil, law and chaos" and more a primordial alignment for me).