I didn't really have any expectations for Quedlinburg and so was extremely impressed when I finally saw the place. The old town has been preserved so well and the whole area feels like you are actually in a medieval town. The streets are full of beautiful old buildings and the layout (and even some of the shops) are straight out of the middle ages.
Of course, the church at the top of the hill is a must visit but I didn't do too many other 'sights' here - I just enjoyed walking around and getting the atmosphere in the streets.
A lot of the people who visit just come for the day (either on their own or in a tour group). I stayed the night and was really glad I did - the old town is much quieter in the evenings and you can see things and walk around without being amongst crowds of tourists.
Also, if you're leaving by train (as I did), make sure you get a good look at the station - it's very cute!