Even though I use Markdown, there are certain things that I prefer the AsciiDoc (or other markup formats) way of doing it. You already mentioned one of them -- the use of underscores. It's non-intuitive that Markdown formats _this_ as this
I mean, they're under
scores...they look like under
lines, for goodness sake. I would have preferred italics use forward slashes and be formatted so that /this/ turns into this
. However, some folks have trouble knowing which slash is a forward slash and which is a backward slash, so I can understand that there might have been some confusion because backslashes are used to escape characters. Using backslashes, you can write: \_this\_ which would appear as: _this_. As it stands now, to underline text in Markdown, you have to use <u></u> or <ins></ins> tags. Ugly.
I agree that neither format is the be all, end all. However, at some point, one needs to pick a format and just use it. If one worries what they're missing from other formats, nothing gets done.