I find quick look really handy, but I know you can't copy text from files without opening them. I wonder if there is any way around this, or if it would be possible to do with a plug-in?
Check this tip:
Select Text in Quick Look Windows
To activate the setting, open a Terminal window and type the following:
defaults write com.apple.finder QLEnableTextSelection -bool TRUE;killall Finder
The changes take effect immediately. To deactivate the setting, open a Terminal window and type the following:
defaults delete com.apple.finder QLEnableTextSelection;killall Finder
It works only from Finder windows. Which I guess is what you're after. Confirmed on Lion.