“Why should one have to read the instructions in order to post? It should be a skill we learn once, and then use forever. It shouldn’t be something we have to re-learn on every single site we visit.”
In the ideal world, that would be the case - and believe me I’ve wished for this many times over the years when I’ve posted on a forum, only to find out my post looks like cr*p because that particular forum software has a different standard for brackets or tags!
But until there is a universal internet standard for freeform text input (i.e. never), different sites are going to implement said input differently. This is where I like SO’s approach, because Markdown is so simple: four spaces or backtick for code, right angle bracket for quote, double angle brackets for a link… a goldfish could remember that ;). And 99% of the time, do you need anything more?
Although I will agree that making links with Markdown is a pain, particularly if you’re trying to add a link in a comment, where the angle brackets seem to be ignored.