How are angels and humans different? It might sound like a stupid question at first, because obviously humans and angels are quite different in many ways... but bear with me a moment.

In one of the answers to my question What does &quot;in the image of God&quot; mean?, the poster says:


  [M]ost Christians interpret it to mean that we share in the spiritual reality with the spiritual beings, and traits specific to God (spiritual traits, since God is Spirit) belong to man. Such things then do not belong to other creatures, which may or may not include the angels.


It then lists four specific traits that humans share with God:


  
  Ability to create
  Reason
  Having a 'spirit'
  Being able to have the indwelling of the Holy Spirit
  


This makes me wonder: How are humans and angels different, in essence?  Of these four traits, angels seem to clearly posses a spirit, and likely have reason.  Are they able to create? Are angels 'indwelt' by the Holy Spirit? (Is it possible for a non-physical being to be indwelt anyway?)

What other traits do angels and humans share, and how else do they differ?

To provide a doctrinal scope, let me say I'm seeking views from an evangelical protestant* view, although I would still entertain answers from other views so long as they are labeled so.

*I realize that even "evangelical" may be too broad on this topic, as even if I specified a very specific denomination, there are likely many views supported within the official view of the denomination--since this is probably not considered an essential tenant of any denomination's statement of faith.  So... I hope this doesn't make the question unanswerable.
WARNING - CATHOLIC ANSWER

A religious sister at the end of the Faith Formation class I teach at was talking about this difference last night.  


Angels are pure spirit, like God. 

"Angel is their office, Spirit is their nature" - St. Augustine (see CCC)  
Angels do not need to be saved by Jesus.

They made their choice to serve God or not before time began and do not have a fallen nature brought about by original sin.  
Angels cannot consume the Eucharist.

Some theologians believe that this, coupled with the vision that Jesus would come to redeem mankind and be born of a woman, is the reason for Lucifer's revolt.


Catechism entry on angels

The internet does not seem to be a good place to source this info, or my google-fu is a little off.  Most of what we posit about the nature of angels is borne of private revelation.  

So take it or leave it, I'll try to improve this answer when I come across the places I heard this information.