Things like Kramdown IALs, for example. Apex can handle a lot of (but not all) of Pandoc's special syntax, including cite-proc and bibliography support, but using Pandoc means you don't get features from Kramdown or mmark. Pandoc is über-powerful, but for 80-90% of simple Markdown->HTML cases, I think Apex is going to offer everything Pandoc does plus the benefit of extensions from processors it doesn't handle, and where it does handle them, you have to choose `--from markdown_mmd` or `--from gfm` --- with Apex you can use both mmd and gfm syntax at the same time with one unified parser.