Supporting every feature of markdown would enable people to insert eye-catching but distracting comments. Besides the features already described at https://news.ycombinator.com/formatdoc, which ones would be most useful without being abusable?
Right now people just paste links as-is, or use this numbering syntax [0] which works but could definitely be replaced with something better.
After that, supporting lists and fenced code blocks with syntax highlighting would be handy, though those are probably more prone to abuse.
[0] Where the link goes here.