So, now we get to some of my favourites - the Negative Quasi-Elemental Planes. All of them are touched by entropy, darkness or death - thus making them perfect fodder for inexperienced adventurers!
"Oooh, the Plane of Ash sounds exotic... uuuurgh*hackcoughsplutterdead*"
Speaking of Ash, the Scions of the Dying Embers (as they call themselves) are a quiet, solitary race. Where possible, they carve out their own little lives on Ash, away from everyone (or rarely, in small groups). Those rare few who head to the Planes do so to really get away from everyone - mostly hermits, or lone farmers. Not particularly conductive to long-term PC play, but good as very rare NPCs.
Dust Genasi are my favourite. On Dust, everything fades - buildings crumble, people desiccate, and items rust. Nothing lasts bar memories and history - and those are a Dust Genasi's bread and butter. Many who take to wandering the Planes do so to hunt out interesting histories - whether its magic items (their creation, ownership, uses in certain battles, etc.), places (many choose a nice ruin or other ancient structure and spend their time researching, investigating, and generally spending a few years settled down into the task), or even people (immortal races and individual creatures can be a fountain of interesting information). Many will travel with other adventurers, and can even be quite social (if a little somber).
Salt Genasi are explorers of rivers and great lakes. While many Planes have their own bodies of water (or, are made of water), they actually prefer the Prime worlds - there, rivers are more changeable, they interact more with their settings, and they can look at the cultures that each river supports - all in all, they're more interesting. As they are quite likely to be visiting a Prime at any given point, they make a great way to introduce players on single-world settings to the wonders of the Multiverse. Kinda dull for PC use, but great if you want to do a specific concept.
And, finally, Vacuum Genasi - Air, with all the life (and everything else) sucked out. While you might expect this to make them dull nihilists (as D&D4e made the "Voidsoul" Genasi), they are in fact simply ridiculously naive about how the universe works. After all, when you come from a Plane where literally nothing happens, you're just as likely to stand in fear of a swooping dragon as you are to marvel at the wonders of a dog peeing in the street. I mean, you thought the Clueless were bad? They've got nothing on these guys. Weirdly, a very handy race to hand to a first-time player - just give them the direction "Everything is new" and watch them go! Also make for interesting NPCs, but only in small doses (and part of the fun is watching them "learn" about the rest of the Multiverse, one experience at a time.
So - final thoughts time. There's a lot of Genasi, and other Planetouched and for the most part? They're pretty cool. They add a lot of flavour and consistency to a campaign, but be careful not to overuse them - after all, a dude made of pure elemental Salt is pretty awesome, but when every other character is Planetouched, it starts to lose some of its effect. Of course, you can use this to really hammer a point home about just how weird the Planes are when players head home (if they ever do) - after all, most people will be the usual races, and it might seem drab and boring to see so many baseline humans all in one place. Just be careful with their use, and Genasi can add a lot to your game.