Commands always begin with \ and are a common subject for questions, at least where a concept tag doesn't fit well.
We could distinguish commands from environments, concepts and packages. centering is a concept, \centering not. \cite means a command but not the cite package, the same applies to the newcommand package and \newcommand.
The backslash might be a problem in URLs, but that could be solved somehow. If necessary, the site could use a tilde ~ instead of \ or do something else.
Or the backslash could just be shown on the tag like those small icons on other tags of other SO sites (like the icon on the [android] tag). Everything is without backslash like entering and URL, but the image of a command tag has the \ on it.
A technical discussion might come later if we desire it.
It would look strange if I omit the \ in writing. Telling that newpage breaks a page and newline does it for the line, while linebreak and pagebreak behave in a slight different way.
Writing a command in a tag the way we do in LaTeX makes it clearer and avoids confusion. A book index would do the same.
One problem if we decide for [foo-cmd] instead of [\foo] might be searchability:
A new user might be tempted to search for \foo and tag seach is actually much more specific than normal search, so a search for [\foo] should imperatively find the relevant tag.
I think it’s quite important to distinguish this from a search for [foo]: the search of the OS X PDF viewer “Preview.app” does this, i.e. a search for “\foo” invariably resolves to “foo” and renders the search results completely useless.