Your quotation and list syntax should work out of the box in most Markdown flavors (HN has a very non-standard and crippled version - basically nothing other than italics, paragraphs, and code/preformatting works.)

Strikethrough and bold are doubled to avoid common ambiguities. Your underline should technically work, but it comes out as an <em> (emphasis) tag, which is rendered as italics in most browsers.