Change the shortcut of cmd+Q for all applications
This should change the shortcut to ⇧⌘Q for most currently installed applications:

defaults write -g NSUserKeyEquivalents -dict-add $(mdfind kMDItemContentType==com.apple.application-bundle | grep -v { |
sed 's/.*\//Quit /g;s/\.app$//g;'"s/'/\\''\\'/g;s/^/'/g;s/$/' '\$@q'/g" | uniq | tr '\n' ' ')

You could also use KeyRemap4MacBook to change ⌘Q to some other key combination or require holding or pressing it twice to quit an application.