The Lua code:
mw.title.new('Wikipedia talk:Articles for deletion').prefixedText
Gives the result:
"Wikipedia_talk:Articles for deletion"
Most users will be expecting spaces rather than underscores, as that is what is normally used in wikilinks on Wikipedia. Using underscores also causes problems if you try to test for equality between title.nsText and mw.site.namespaces[namespace].name:
mw.title.new('Wikipedia talk:Articles for deletion').nsText --> "Wikipedia_talk"
mw.site.namespaces['Wikipedia_talk'].name --> "Wikipedia talk"
Having these two commands produce different text seems counterintuitive to me.
I've posted a related issue at Bug 56216.