Almost right. 5 is indeed the top but the bottom is 0, reserved for books so poor that I couldn't be bothered finishing.
As for the differences, that is what makes the world go around. Have found several really good books based on commoners (as opposed to critics) ratings. I often find them more in tune with what I like than critics writings/ratings. Especially in moviedom, have been burnt too often by the latest blockbuster superpic (anybody remember Independence Day - worlds most overhyped and worst SF movie)
For me when rating a book the biggest factor is probably how well the story, or the characters engage me. For example I rated Einstein Intersection a 2, not because the writing itself was poor just that the story it told was incomprehensible to me. Conversely Paladin is probably only moderately well written on a technical level and certainly breaks no new ground within the genre. However there was something I just identified with in Ista and the others that swept me along and in my eyes - and as it won the Awards others eyes too I suppose - that lifted it above the pack.
Don't know about posting my 'journal' online, but an I've been reading thread might be a good idea, just might be a while before I contribute my first one as I am currently reading Last & First Men which while interesting, at least initially, tends to put me to sleep after 20 or so pages - or that might just be the long hours at work