I usually read manga more, specially because it's less time consuming and detailed. I may drop some shows that I think are good but just aren't my cup of tea, I'll usually leave them unrated too.
Since most series I watch/read are ranked 7 or above, here is the reasoning I use to rate series, more or less:
10 -- I had a blast watching/reading it and I recommend it.
9-8 -- Fun to watch/read and enjoyable, most series I rate are here even tho they might be a bit boring at times or have some flaws.
7 -- mostly guilty pleasure series, simple shows such as some slice of life and some ok animes also go here.
6 -- Probably finished for the sake of finishing. Not bad but no good either.
5-4 -- I may have finished it but it was most likely dropped, wasn't enjoyable.
3-2 -- I regret watching it, waste of my time.
1-- Probably dropped it on the very beginning of the series, awfully bad with no redeeming qualities.