That you have to keep each and everyone of your mounts/pets “fed”, each with a specific food/fuel you need to procure. If you don’t feed them they might die/stop working.
You’d probably reduce your mount to only a few. What’s the one you’d keep?
Edit: And what do you think it “eats”?
My Jeweled Onyx Black Panther because its literally an animated rock that doesnt need to be fed.
It also flies.
Sandstone drake, because it flies and can carry someone.
Just like a hunter in classic.
The blood and flesh of my enemies!
“But it’s a horse.”
Evolve or die!
Right now I only have a snowy gryphon.
I bet I’d have to feed it any animal with white fur.
Hm… i basically only use one ground and one flying mount per character anyways.
Druid doesnt use mounts, got travel form.
Nelf hunter uses moonsaber and hippogryph.
Worgen deathknight uses garn the wolf and the deathwing mount, at least until i can manage to get the frostwyrm mount from icecrown raider.
Worgen warlock uses dreadsteed/felstalker depending on spec, and twilight drake.
Vulpera shaman and vulpera monk will be using hyenas and wyverns most likely.
I’d be stuck with my Ghastly Charger and Swift Spectral Tiger, pretty sure ghosts/spirits can’t eat anything.
I’ve always thought the paladin Chargers (not the warhorse, but the upgraded charger) was a spell. More of a mystical, ethereal creature that is less “real” and more “magic/Holy.” Thus it probably doesn’t require sustenance.
I mean beyond the piety and righteousness of its user.
The magic carpet, because it doesn’t eat.
Favorite movie.
Made my day. Thanks!
H
Edit: Not meaning to derail but I’d feed my Time-Lost whatever it needs. The Raven class mount, I’d end up feeding it the fallen or ambushed enemies that I run across. 
My Kodos, they eat grass and it would really be nice if they cut the grass for me.
I am fond of my undead dragons, but I have no idea what they eat. Pretzels? Everyone likes pretzels. I could really use some pretzels right now.
Lightforged Warframe. Small-ish, flies smooth, looks awesome.
Another character of mine does maintenance on it.
As for what it “eats”… holy-powered battery crystals?
Ah ha! Spectral Saber doesn’t need food. It’s already dead! 
How do you “feed” a kite? 
My ivory raptor, because it’s my most treasured mount. I still remember getting it shortly after reaching 60 in vanilla.
She can eat some of the meat from whatever I just killed. That should do just fine.
My Obsidian Worldbraker as he is a Drake/Dragon and I’m sure he can take care of himself when it comes to food when I don’t need him. I would have no idea what a growing World Breaker eats.
String and tears of little kids that they lose them.
Maybe that was just me. Hmm.
H
As I am my own mount, I want to be fed. People just aren’t willing to let me eat their various pets, though. 