My rear derailleur was working fine up to the point when my shifting cable snapped. I don't think it needs to be replaced (although I'll try ridebikeme's test later today).
Yes, I changed the cable housing too (cut to the same length as the old housing).
I did change gearing in the cassette from an 11-28 to 12-27 but haven't really touched the B screw yet 'cause I'm not entirely clear on what it does.
It's possible that my cable's too loose, but I've tried to get it as tight as I can. Is there a trick to this?