Interpreting parables

People I know have a "love/hate" relationship with New Testament parables.  Some people find them easier to understand than others.  In spite of this difference in opinion, there is still real commonality in the way most people view parables:

1. There is only one key point in a parable.
2. They are illustrations of heavenly/spiritual things with common things.
3. They are straightforward and unsurprising

This understanding of the nature of parables was put forward by Adolph Julicher in 1886 and is now commonly assumed to be true in many Christian churches.  But is it really a helpful and accurate paradigm?  Recent scholars like Craig Blomberg and Kenneth Bailey have demonstrated that it is not.  Consider, for example, the parable of the Prodigal Son.

1. One key point?  The "prodigal" tells us of our sin and the way a sinner may be freely reconciled to God.  The father tells us of God's extravagant love and grace.  The older brother depicts the dire situation of the legalist; they rely upon their goodness, cannot stand grace, and stand outside the loving fellowship of God.  It seems that there are a number of key points to the Prodigal Son, not just one.
2. Are all things illustrations of heavenly things?  Surely, God is being illustrated by a "father".  But how far do we push this?  Does the pig stye become something spiritual (e.g., the existential experience of a sinner before conversion?) or is it simply a part of the story?  Does the road back home refer to the inward journey of the soul?  What about the fact that the calf was fat?  Does it refer to the sacrifice of Christ in the fullness of time?  Caution needs to be taken in assuming that everything has a spiritual meaning.  Characters in a story should be allowed to use props without us drawing massive interpretive conclusions.
3. Are they straightforward and unsurprising? Isn't the father's hiking up his garments and running surprising?!  What about the fact that he actually gave his son his inheritance early?!  The surprises are there for a reason.  They inform us of what we are to read into and demand our interpretive focus instead of unsurprising elements like a road.

[I derived these insights from SM Baugh's class "Gospels and Acts" at Westminster Seminary California]