I'd think that the most important thing, more important than getting directly paid royalties, would be to be clearly and prominently identified as the songwriter either in the YouTube video itself (preferably in an intro caption) or in the little blurb that accompanies the video (ideally, just under the title).
This matters a lot more in cases where either the general public isn't familiar with the songwriter or where the songwriter is well known but the public has no idea that he or she wrote the song.   
I don't know if there's any handy recourse with YouTube and similar sites.   Suing people would probably cost more than it would be worth (not to mention scaring people away from helping "share" your songs with the world).   Posting a comment has, well, the limitations that you've mentioned, though Poppagator's suggesting is spot on.   If there's any way to contact the people who've performed your songs and posted them on line, I'd contact them and ask them to give you proper acknowledgement as the composer.
Maybe you should post your own recordings of each of these songs -- or portions thereof -- on YouTube with clear, hard-to-miss captions that point out that you wrote each of those songs (and telling people where to buy the CD.    Then when someone plays one of the other people's versions of your songs, your videos' icons and captions will show up in the sidebar.   
People often check out the "related videos" in the sidebars, including checking out multiple recordings of the same song.