@HolyPhoenix The way I see it, it's an improvement over BBCode because it's still readable even without parsing.
Take the following example short paragraph, in Markdown, followed by BBCode:
Steve walks *warily* down the street, with brim pulled **way down low**.
* Ain't no sound but the sound of his feet
* Machine guns ready to roll
Steve walks [i]warily[/i] down the street, with brim pulled [b]way down low[/b].
[*] Ain't no sound but the sound of his feet
[*] Machine guns ready to roll
Markdown is definitely more readable, hands down.
_ already map to "emphasis" in most peoples' brains, so it's not that much extra mental processing, whereas with BBCode, you're having to parse out square backets, etc.