Clearly mark the tag boundaries
Support copy and paste of tags
No hierarchy, classification, or anything else that disrupts the cognitive process
Ability to edit tags like text
Search previously tagged items with an already-typed tag without disrupting the tagging
Enough whining. I'm going to try to be constructive by listing what are the behaviors that will make me fall in love with a tagging thingie:
clipped from: yanivg.blogspot.com