Both, really. If just about every fanfic plot and setting has already been thought up, then what's the point? Equestria Daily takes about 30 fics a week at minimum, and these are supposedly the best. If I can't even know what the basic locations and ponies look like, then how can I write anything decent? I'll be swarmed by accusations of sloppy writing and not even watching the show. I don't want to sound like an emo teenager, but what can I write that hasn't been done better already?
I really don't think every plot and setting has been done before. Sure, there's a ridiculous volume of fanfic, but 80 – 90% of it is terrible (thank you,
Sturgeon) and huge quantities of them are variations on popular themes. So there's no guarantee that any plot you come up with has been done before... and even if it
has been done before, it's probably executed much differently than how you would do it.
For example, I'm writing a fanfic whose premise boils down to "Derpy sees stuff that nopony else can see, and secretly uses that knowledge to save Ponyville from eldritch horrors." After publishing the first few chapters, I discovered that at least two other fics had already used that premise. But one of them was a one-shot and had a much more cynical tone than my story—and the other fic was a
Magic: The Gathering crossover (and aside from that, leaned on popular fanon more heavily than my fic does).
Or heck, look at
A Delicate Balance. It's a Twilight/Applejack shipping fic—that may not be the most popular pairing in this fandom, but it's hardly unexplored territory. But Jteeth's execution of the idea (keeping everypony in-character, portraying the less-pleasant aspects of a relationship) makes the story stand out anyway.
As for describing how stuff looks, I'm not sure that's truly necessary. Good writing should appeal to all the senses (and the written word lets us get into character's heads better than other media do) but most writers lean too hard on the visuals, if you ask me. So if your visual descriptions are somewhat sparse, you can make up with it through your descriptions of the other senses.