How can you tell if a story you just read was just decent or if it was really good? Well, usually reading the whole darn thing in less than 24 hours is a good hint. Bonus points if it isn't finished yet and you're fighting the itch to make an obligatory "When's the next update?" comment. The Changeling of the Guard is one such story, at least for me. And with over 80k words, the whole reading-in-a-day thing is a pretty big hint that I enjoyed it.

Changelings are one of those more peculiar story premises. Changeling stories - as long as the spelling and grammar is solid - are rarely bad stories. The concept in itself is good and comparatively easy to spin into a story, and exactly that's what makes it so hard to do something truly remarkable with it. One changeling / several changelings / all changelings are living in disguise in Equestria (usually after or just before the failed invasion). Then things start going downhill for some reason (in many cases due to said invasion) and their disguise is in peril; or they only just arrived and already are at rock bottom.
...well, the protagonist of our story is an excavation drone, so one can only guess how much deeper he will yet be able to dig himself.

What makes vdrake's story stand out is a mix of engaging characters, well thought-out backstory and a protagonist who is trying to learn even as he remains adorably clueless to many parts of pony society. It's not that he doesn't know anything, it's that he actually has quite a bit of life experience and all of it comes from a monarchic system of expendable workers, rigid traditions and blunt, efficient communication, with especially the last one being quite substantial to the story. The continuous first person perspective conveys this perfectly and fits into the story way better than in most others I've read.
The backstory is immersive while not overbearing at any time except maybe the first chapter/prologue, and does not hinge on Chrysalis being a bad leader for a change. The humor is lighthearted and easygoing with some real gems strewn across the story, and well deserving of the Comedy tag. The actual plot is well paced, believable and not afraid to go into darker and seriously serious territory for a couple paragraphs before we are back to jokes and bugstick. And all the characters are immersive, endearing, believable (common sense, it's real), and even downright quirky in some cases. A changeling meets an entomologist. Need I say more?

All in all, a good story that I'd easily give 9.5/10 points. It's still continuing, and doing so at an even pace, though I should probably mention that it does so even slower than my own stories. Don't let it keep you away from the story, because that would be a shame, but be prepared for the withdrawal symptoms once you've reached the end of the last chapter.
You know, maybe if you get more active in this group you'll find more of those excellent stories to tide you over.

This house, Shining decided, looked like it. I wasn’t sure his judgement was currently sound, but I rapped politely on the door. Shining made a valiant effort to stand, looking as though he expected to be hit with a cart any moment and had resigned himself to the impact.

The door burst open and a pair of ponies bustled out, one either a very light grey or white, with violet stripes in her mane, and the other a more solid blue. They looked to be very concerned, and I could only make out a bit of it.

“-have you been-”

“-worried sick-”

“-smell like a distillery!”

“-what were you thinking-”

Shining held up a hoof, partially to block the questions, perhaps to forestall further questions. “Graduation is tomorrow.”

I stared. “...You didn’t tell them?”

“Ssssurprise!” He slurred, nearly falling forward.