I went to Regensburg and nearby Nuremberg at the same day. I must confess my impression that the latter completely overshadowed the former. Both are mediaeval cities but while Nuremberg was erased during WWII and completely reconstructed afterwards Regensburg still preserves its original buildings. Is this a reason enough for its inscription? Dresden is a good counter-example.
Well regardless of Nuremberg, I can say that Regensburg is a relatively dull town unlike many other colourful mediaeval towns in Germany (or even in Bavaria). Still there are two highlights: the stained glass at the local cathedral and the interior of the town hall, which I would really recommend visiting. It features the largest undomed hall for its time without supporting columns, which was a major engineering achievement.