Copy spotlight comments into a text file?
You can use mdls to list the metadata attributes for a specific file. Here's how to show the comment for an imaginary file called myPhoto.jpeg in the Pictures folder:

mdls -name kMDItemFinderComment -r ~/Pictures/myPhoto.jpeg


And here's how to find all files on the computer that have a comment:

mdfind "kMDItemFinderComment=*"


Here's how to dump all comments for all files:

mdfind "kMDItemFinderComment=*" | xargs mdls -name kMDItemFinderComment -r


And dump the names of all commented files, along with their comments, as a CSV:

mdfind "kMDItemFinderComment=*" | xargs mdls -name kMDItemFinderComment -name kMDItemFSName -r | xargs -0 -n1 echo | awk 'NR%2==1{printf "\"" $0 "\","} NR%2==0{print "\"" $0 "\""}'