Copy Text from Quicklook
Running

defaults write -g QLEnableTextSelection -bool true


and relaunching applications works for me. defaults write com.apple.finder QLEnableTextSelection -bool true only applies to Finder but defaults write -g QLEnableTextSelection -bool true applies to all applications.

If it doesn't work, see what Quick Look generator handles plain text files:

$ qlmanage -m|grep public.plain-text
  public.plain-text -&gt; /System/Library/QuickLook/Text.qlgenerator (555.0)


If it is not Text.qlgenerator, try to delete it and run qlmanage -r.