I was going to read through everything in this thread, but quite frankly I don't have the time, but I will address some of the thoughts that I had while reading the first page and a half (oh god this is probably going to become a giant response).
First off, I did a lot of judging for pony-ness myself since i've gotten into the pony music thing, and in the beginning, I disregarded a lot of instrumental tracks simply because I didn't see how it made them pony. But as I went on, I found that I could really connect with the emotions in a song, whether it had vocals or not. Based on the title, and sometimes the art or description in the video, the music would paint a picture and project emotions into my head. I find that I can actually connect with pony music on an emotional level better than I do with, for lack of a better word, normal music. It gives me a general area, a world with characters, which guides my emotions, and I feel I can understand what the composer felt while writing the song. I have a few examples from the top of my head. Exhibit A: The Astronomical Astronomer's Almanac to all Things Astronomy by Makkon and PinkiePieSwear
http://www.youtube.com/watch?v=SLG9aEZa_egDisregarding the silly Spike sneezing sample at the very end, this song fills my head with feelings of awe, with pictures of galaxies and celestial(hah) wonders. I can picture Twilight and the rest of them gazing up at the stars at night. One might argue that it could project images of anyone gazing at stars, but because of the title, my thoughts are guided towards ponies, and because of that I consider it a very pony-related song. Exhibit B: Everfree Compromise by Pogosama
http://www.youtube.com/watch?v=6B2Q28FPTb4I'll admit I didn't find this to be overwhelmingly pony, though it qualified when I first heard it. It mixes elements of both instruments and electronic music, creating a feeling that is consolidated with the title and image, showing Octavia and Scratch together, with the word Compromise in the title, giving a feeling of resolution to a difference or problem. It's also a very peaceful song, which emphasizes that. The feelings were magnified when I heard the song at the end of The recorded version of The Vinyl Scratch Tapes first episode. It went so well with the story, where Scratch makes Celestia feel bad because she never apologized to Luna for sending her to the moon for 1000 years, and she leaves to apologize, and Octavia is freaking out the whole time thinking Celestia was going to banish them, etc... and the narrator wraps up the story with this song playing, and I was overcome when I first heard it. It brought tears to my eyes for some reason. That anecdote went on way too long, but the point is that I associate that song with the feelings it gave me at the end of that radio-play thing, and that made it double pony for me. Exhibit C (the last one, I should think): Everfree Forest by Pinkie Cake
http://www.youtube.com/watch?v=in85-hCJZCMThis is an example of an instrumental electronic song that I find to be quite pony. It creates this odd feeling that makes me feel like i'm in the Everfree Forest, a quirky and dangerous place. It's similar to the feeling I get from listening to a certain remix of the Viridian Forest theme, called Out of Antidote if I recall correctly. It seems sort of off, like you're poisoned from a Pokemon battle and you have nothing to heal it with. Also a small amount of creepiness. Anyway, that's the same vibe I got from this song, so I consider it pony. For me, it's on a song-by-song basis, but most songs I find via EqD posts usually make the cut. The point i'm trying to make is that a song can be pony for me if the music in combination with the title, and sometimes album art, can define pony-ness for me.
As far as samples, I enjoy them. They can enhance the ponyness for me as long as they make sense in context of the song. Obviously this is harder to judge with stuff like dubstep, but you get the idea. But keep it original. There's nothing wrong with a "yay" or a "LOUDER" for me, but I can only take so many of them.
Although most of this is probably more relevant to the first page of discussion than the current one, I wanted to contribute. And I apologize for the great length of this post, I tend to ramble when I have a lot of thoughts on something.
TL;DR, the emotions make a song pony for me, and those emotions can be guided greatly with a title, and sometimes album art. And samples in songs are okay by me.