Actually, the 8+ thing is pretty smart. They avoid offending anyone by leaving the benefit of the doubt. At one point, judging the difficulty is subjective and differs person to person. It's not possible to say whether Beethoven's Hammerklavier or Liszt's TEs are easier (taking the example from j_menz's bottom tag thingy). So this sight just sticks them all under one category, 8+. That 8+ could be a 9 or it could be a 12, but it's hard to judge the difference between a 10 and an 11 so why bother?
The issue I have is how some pieces are rated as 8s and even 7s when they could easily be 8+. Also, there are some pieces that are rated as 8+s when they probably aren't much higher than 8.
For instance, Chopin's Barcarolle? Rated as an 8 but most people consider it to be just as difficult if not more difficult as the ballades. Perhaps not as difficult as the 4th, but respectably close. My teacher actually ranks it higher than the 4th ballade.
And some of Grieg's lyric pieces, in particular, wedding at troldhaugen. Rated as a 7, but I've always considered it to be (while not difficult) somewhat more difficult than say, Beethoven Moonlight or some of the Chopin etudes (all of which are at least an 8, which I agree with. Which ones are 8 and which ones are 8+ I don't agree with so much...)
Henle's ranking system is somewhat more detailed, but somewhat more disagreeable. Their system goes from 1-9 with 5 being comparable to the 7 on this site.