I'm not posting this because of any recent events that come to mind, but I [and I'm sure many artists] have had problems with things like this in the past, and I think it bears linking.

Don't complain about free ice cream. [<link]

Sometimes artists promise something and intend for it to be free. To use myself as an example, I still have a pretty extensive list of names for those free sketches I was going to do last year. I still intend to do them [and I finished all the ones I was paid through donations to finish], but life/paid work/everything has forced me to put that project on the backburner.
Thankfully, no one who requested a sketch has been entitled/indignant toward me about it!

I'm merely saying, if you request free stuff from artists, make sure you realize that artists have lives, too. Unless money changes hands, we are not working for you. If someone offers you something out of the kindness of their heart, thank them. If they end up not able to do it [soon or ever] for some reason or another, remember: it's the thought that counts. Thank them for their good intentions and move on. You can still enjoy everything else they put out in the meantime, even if it's not specifically for you.

And if you like their work enough and can afford to do it, consider commissioning them. It makes us feel really great to know that there are people out there who would pay for us to do what we love to do and for many artists, it keeps them fed!

The other thing I notice with free art offers is when the artist DOES do the work and then gets complaints that it's not what the requester wanted.
Deal with it.
As said above, you're not PAYING for it, you lose NOTHING. Just because you're a little disappointed that the pose isn't exactly what you envisioned or your character has too many spots on the left side is NO reason to look that gift horse in the mouth. The artist took time out of THEIR life to give YOU a gift. Regardless of the fact that it was requested, keep in mind that it was something they had no obligation whatsoever to complete and they chose to do it expecting nothing from you in return.
But in return, you give them criticism and complaints? That's just downright rude.

In conclusion, mind your manners, FA! Don't act entitled, don't take advantage. Be nice, be understanding and aside from the fact that it's the right thing to do, I assure you that any artist will be MUCH more willing to find the time to draw your free art if you GIVE them time, rather than demanding it of them.

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[As a side note, this entire concept is why Rarity became my favorite Friendship is Magic pony after the Suited for Success episode. She became the most relatable to me. :P]