I can name three problems with it straight-away: 1) HTML, 2) CSS, 3) JS.
The original post is about a decent language, not the ubiquitous ones.
Honest question: Why?
I think I can answer that. IMHO Liveweave has something most of the other JS "playgrounds" do not have. It has context-sensitive code-hinting for core JavaScript and jQuery...also for CSS/HTML/HTML5/CSS3. Personally, I think this makes a huge difference, at least for me and may be some others.