Nice post, but there’s a few things in there I thought I’d address as a seller with some info from the other side 
Firstly, there are much larger studios purchasing assets. Granted they may not be quite technically AAA, but ARK: Survival Evolved was almost entirely marketplace assets when it launched, Conan Exiles uses a LOT of marketplace assets, and apparently Player Unknown’s Battlegrounds also has a lot of assets. These aren’t quite AAA games, but they are far from indie or hobbyists, each making huge millions of dollars and being made by teams of 30+ people. For the most part, customers usually are indies and hobbyists, but it’s not all of them anymore.
Secondly, the reason a lot of assets have more flashy names is because Epic actually have a rule for asset names that is “no generic names”. In practice, they rarely seem to apply it (as there are generically named assets all over the place) but I think a lot of sellers get caught in a trap because they think that means that they can’t use a fairly common sense name.
Regarding the description - there is definitely a limit. We are allowed, at a maximum, 2000 characters (or it may even be lower now) for the product description, and any html code for links (such as the video) is included in this, meaning that in practice you end up with much less. It’s a little easier now that sellers are able to directly edit this information, but it’s still not a very large amount of text. That said, most products that I see on the marketplace don’t even get close to using this 2000 characters and you are right - they don’t cover what the products actually do, include or anything useful. Honestly, the 2000 character limit is very frustrating, particularly with larger packs. Take for instance my own Multiplayer Survival Game Template - it’s impossible for me to get even close to covering all the stuff included in the template in that short amount of space, let alone linking to the video, the dedicated website I have, etc. Very frustrating!
As for showing the asset - Epic are supposed to be enforcing that all assets have one in-engine screenshot at a minimum. Unfortunately, Epic take control of all the screenshots at the moment, so if someone gets through submission without an in-engine screenshot then honestly, the fault is on Epic at that point IMO. On the flipside, the requirement for all assets to have screenshots, including sound assets, is stupid haha.
When it comes to thumbnails and including text - I think the reason a lot of sellers here are avoiding text is because Epic have, at multiple times, changed the dimensions of the images displayed at various points on the marketplace, resulting in text getting cut off when items get randomly featured and stuff like that. Whilst text increases your CTR, having text that is off center and runs off the edge of the displayable area makes the product look very unprofessional, which hurts your product far more than the text helps it, if that makes sense. As a result a lot of people have just stopped using text, at least that’s my feeling on it. As with a number of these issues, sellers are currently having to dodge a whole lot of stupid situations put forward by the marketplace still not really being up to scratch as a platform, and inconsistent/unreliable/changing situations with the content and how it is presented.
Videos is the biggest point I 100% agree with. So many assets have no video, or horrible 7 second videos with no audio. Videos are your biggest selling point once someone is on your page, and not putting some time into on really hurts your sales. That said - the 7 second videos and the 20 minute videos are bad moves too. Even Epic themselves recommend a trailer of about 2 minutes for your main video with longer videos for tutorial content and such. 2 minutes is a good amount of time to convey an idea of your product and why it is great, without it dragging on and losing momentum.
I don’t agree, however, with your point about audio assets using a video with content in it - but that is more from the perspective of having seen a lot of customers who ignore disclaimers saying “this content is not in the pack”, buying it and getting angry and leaving ranty comments and such on the product pages. In theory I think it’s a great idea to have a video for audio assets like that, but in practice, even if only one or two people leave those ranty comments, it hurts your product overall. Audio assets SHOULD have, at minimum, a soundcloud or something along those lines with a decent preview of what is included though.
And finally the tutorials - definitely. The fact that more marketplace developers don’t do some of the basic tutorial stuff still boggles my mind. I’ve been investing a lot of time in my assets, making tutorials covering how to integrate OTHER marketplace assets into my own. This is definitely not something everyone needs to do, but I can say first hand that the more tutorial content you have out there, the happier your customers will be and the less they will hassle you for help, even with very complicated assets like entire game templates. But at a base line, covering how to set up your product in a project and get it all working should be a minimum for coverage, and a video tutorial is infinitely more helpful than a written document. However, it’s also important for creators to make sure the video is watchable (decent audio, nicely paced (ie. not too fast for people new to the engine), etc.).
Anyways, just my two cents offering a little counter to explain some things Nice post.