* It's not standardised
* I don't like using it
I think the two big advantages of markdown are simply:
* It's readable without having to parse it
* A lot of people do like using it
Replacing it with HTML, as useful as HTML is for its intended purpose, sounds like an incredibly bad idea. There's a very good reason why markdown won: its readability. Except for tables, but that's why people don't use them. It's not meant to be suitable for every conceivable purpose, it's meant to be readable.
Using a rich text editor for it sounds like a bad idea to me, because you don't know what the raw text is going to look like. And yes, I do type > for blockquotes. It's not very hard to do.
Should we have a single standard for markdown? The lack of one seems to be the primary disadvantage of markdown, but personally I'm not sure it really needs a standard. It's pretty simple and I don't think it really needs to be portable. But it would certainly be nice to have a standard.