I have been using OmniOutliner since 2004, having migrated from Windows and EccoPro. My thoughts on the questions:
# 1: Yes, I would need multiple columns of data and all the column types. But the no of columns could be limited to 3. No need of summary options.
# 2: No need of formatting support, but being able to "bolden" the text would be fine though.
# 3: No need of attachment support
# 4: Editing capabilities would be a must
# 5: Must be syncable with my desktop, preferably via idisk, like Omnifocus.

Keep up the good work on applications!