Number one on my list, too, ML!!! I've written volumes on the subject, had two letters published in Cruise Travel Magazine on the dreaded single supplement!!!
My first cruise on NCL, the Southward, I had an outside cabin and it had just a twin bed! It was way forward, there were 8 cabins, no extra pricing, same as other categories.
What really got me back then was the lines that had single cabins but still priced them 150 to 175% over other categories!
I still hate it, but at least now if you pay that extra, you can get a decent location/size cabin before you paid extra and got the really awful locations!