if you just want to assign a score, based on the keywords you might want to have a look at this thread http://rapid-i.com/rapidforum/index.php/topic,8638.0.html
If you want to find the words automatically, you can do standard text mining on them. The trick is, that you can cluster the documents. Afterwards you can use the cluster information as Label and do a feature selection on them. Thus you can get the important words per cluster.