Comment by Carpetsmoker

Emojis are fundamentally different, as they're essentially just codepoints being displayed. That's fairly easy to implement, as every system has been doing that for years. What you're proposing would require massive rewrites of a lot of software.

I'm also not sure it would even be better; I've never encountered a graphical editor that didn't make me want to throw my computer out the window; stuff like text becoming bold when I don't want it, adding to a list (especially with copy/paste), etc. all tends to be quite annoying and stuff I need to think about. With Markdown, I don't really need to think about the formatting, not more than regular typographic formatting anyway (paragraphs, punctuation, etc.)