Recently you had an all around update on comments. Great! Finally!
Yet it wasn't anything like reporting comments, or giving us a way of ignoring certain people. But at least a step in the right direction. But one thing in particular is disappointing here:
It seems you did the same mistake again as you did with last_activity.

In terms of last_activity:
If a user edits any comment, commented_at of that user would change.
But you'd have to sync ALL comments ever written by that user because you have no chance knowing what or how many comments were edited (or in this context unfortunately "commented"_at).

In terms of comments/recent:
Again, editing a comment isn't well-thought-out. comments/recent wouldn't change the position if a user edits a comment. Given the fact that last_activity changed I'd expect that comments/recent updates as well and puts the updated comment at the top.
But it doesn't, it stays where it was before, it orders by created_at only.
That means with comments/recent I can only sync comments at a specific point in time. It's basically impossible to sync once and update as needed.
If comments were changed (and I'd have to assume constantly that's the case) I'd have no idea how many comments were changed and I'd have to call all 141640 comments again (worst case) to check.

And why is it that replies aren't in this endpoint? Don't they count as comments in this instance? I feel your wording is highly inconsequential on several occasions leading to issues on where the API goes.

Then there's the issue with likes. Liking a comment changes that comment as well indirectly. But I have no way of syncing/checking that either efficiently.

You seem to create one endpoint after the other to do basically the same thing just with different responses but the same issues.
That reminds me of v1. I'm not seeing the intricate cogs of your API from within but I see it from the outside and what I see is you're doing the exact same mistakes again you did back then with v1...

All we need is a simple way of getting either all or a specific user's comment objects (where replies are considered as comments) and an endpoint to check for comment changes with the changed comment's ids returned, not the timestamp of the last change. The latter is a completely useless information.
Whereas a comment is considered to be changed when: (un)liked, replied to, edited, user rating changed (where possible), or even created.
Endpoint: comments/changed
OrderBy: updated_at
Page and Limit as usual
Returns:
[{
trakt_id: n1,
likes: m,
replies: m1,
user_rating: n2 || null,
created_at: xy1,
updated_at: xy2,
parent_id: 0 || n3
}, ... ]
(Being nitpicky as I am you could then support /comments/[:ids]/* endpoints to lessen the requests, that's what I would do in your position at least)

How would you go about syncing comments of a or all users with your own API as of now? Including updates.
I don't see how that's possible without missing some crucial information or hitting your servers with redundant requests, doing twice or more work on any side involved.

A good amount of shows have canonical versions of their episodes uploaded on YouTube.

It'd be useful if there was a way to "link" episodes with those specific YouTube videos.

This unique video ID (the part after /watch?v=) would ideally be exposed through the backend for API clients. As for the website, instead of embedding the video, a simple link would be fine since I don't think you guys are interested in turning Trakt into a viewing platform.

The reason I'm interested in this is, it's always been a hassle to keep track of your subscriptions and the content you care about on YouTube. It's a big mess. And with the introduction of YouTube original shows, serialized content is only going to grow from here. This change will enable devs to create browser extensions that implement Trakt directly onto the YouTube interface, allowing people to scrobble and track a lot more shows automatically. It will also potentially bring thousands more users to Trakt.

The implementation would be as straightforward as adding a database field for the YouTube video ID. Of course you do have to update the interface and the API endpoints to support the new field.

Some example links:
https://trakt.tv/shows/h3h3productions/seasons/2016/episodes/11 -> https://www.youtube.com/watch?v=RMICTAl4LCg
https://trakt.tv/shows/open-haus/seasons/2016/episodes/32 -> https://www.youtube.com/watch?v=xmHklWqagKc
https://trakt.tv/shows/on-the-spot-2014/seasons/6/episodes/6 -> https://www.youtube.com/watch?v=-1NVV7ufTNE

I honestly don't even know if I'm describing this correctly but I shall try. I don't always watch shows from the first episode, especially if watching on TV. For the past month, when I check into a show that I have watched previously, if I check into an episode that aired BEFORE the episode I just checked into, it always shows the episode AFTER the one I checked into long before.

For example: I checked into The Andy Griffith Show episode 8x24 then started watching the show again from Season 1 and checked into episode 1x01 but my Recently Watched On Deck still shows 8x25 is the next episode I'm supposed to watch meaning I have to check on the episode, go back to Season 1 and pick the episode from there. It will constantly show 8x25 as the next episode to watch until I get to that episode which is about 200 episodes away.

Another example is The Great British Bake-off. I previously watched Season 5 but my PBS is now showing Season 4 so I checked into episode 4x01 but my On Deck is showing the next episode I need to watch is 5x11.

Example:
Deck shows: http://i.imgur.com/4Z9Ik0m.jpg
Recently Watched shows: http://i.imgur.com/u99FtT7.jpg

In my opinion, it would be much easier to use the Recently Watched On Deck section if it would always show the next episode (if it has already aired) that comes after the one you've recently watched/marked and newest one you've watched from whatever season is newer. Especially if it's a daily show like a news program, soap, or talk show that some people don't watch every episode of, but watch on a semi-regular basis.

You're going to love this idea. Ok, so we're human, right? We have 24 hours in a day, 9 of those are spent working, and we get about 2 to watch a movie or some TV. We've all got way too many movies on each of our Watchlists, and many us, by the time we get around to watching some of these movies we added to our Watchlist, we've totally forgotten why we added the movie to it in the first place (why we were interested), especially when you added it for some reason like, a friend recommended it to you, or it's by a certain director you've heard was good, etc.

So my feature idea would be: let us associate a little note with the action of adding an item to our Watchlist. And make it easy to see that note while we're browsing the Watchlist. It'll help jog our memory why we were interested in watching the movie.

I don't think anyone has ever done this.

We can already navigate through seasons/episodes by clicking the '<' and '>' arrows on the covers at the top of pages such as this: http://trakt.tv/shows/archer/seasons/4

I would appreciate this function to be available in the thumbnails of episodes and seasons as well. IDEALLY, the '<' and '>' arrows would appear on the left and right sides of the thumbnail only when the mouse hovers over them, so that it doesn't obstruct the image until necessary.
Another alternative (though it's less beneficial) is to only show the '>' arrow at the bottom of the thumbnail between the 'Manage lists' icon and the 'Rate this' icon, since that's all the space available when viewing thumbnails in any "Personal Lists." However, there is plenty of space for both '<' and '>' icons in the same spot if viewing the thumbnail on most other pages, just like in the link I provided above.

The reason I feel there is a need for this is mainly due to the 'Progress' page. When clicking the '✓' [Add to watched history] icon on an episode, the episode remains there, which makes sense. Because if it were to suddenly skip to the next episode, then we would have to go through an extra hassle to get back to the episode just watched if we wanted to leave a comment on it or had forgotten to click the 'Add to collection' icon first.

Thank you for your time and all you guys have done so far. What you guys have built and are building on is great and is definitely appreciated.