As I mentioned, there are features that the markdown already covers, and it should remains like that. It’s absolutely fine that discourse has a “preferred” formatting language and markdown is absolutely fine (I am coming from a developer background, I lost count of the amount of readme.md that I’ve wrote 
)
But most users coming from other forum software, migrating or being introduced to discourse, will find some features missing.
Namely:
[u][/u]to underline[center][right][left]to align text[size]to change the of text[font]to change the font face[color=COLOR]to set the colour of text[bgcolor=COLOR]to set the background colour of text[highlight]to highlight text[small]to make text… small[aname=NAME]to create an anchor in a document with a name[jumpto=NAME]to create a link to an anchor[img]create an<img src='something-something.com/image.jpg'>html tag, which is not the same as the upload of an image locally. Lots of self hosted don’t allow the uploading because of disk space concerns. This is another topic I would like to expand upon but it’s not the place here
I can agree that they might be limited use tags, but the fact that there isn’t a visual aid (button or autocomplete when someone type a [) it’s a straight worse user experience for communities and users coming from other major community software, legacy and modern.
I am aware of a plugin (or component, can’t recall) that provided that auto-complete feature but it’s marked broken since quite some time now unfortunately.