Now that OneNote is being actively developed by MS for all platforms, O+ needs to be more than just a pretty skin on top of ON notebooks. It needs features that don't exist in ON to stay relevant, especially given the price.
My personal take:
1) Ability to store files in other services is a very big +
2) Ability to export PDFs from iPad is a small +
3) Inability to move pages to other notebooks is a -
4) Inability to rename attachments is a big -. When using Office lens or Scannable to send PDFs to Outline, I always have to change the nondescript templated name of the file later.
5) Inability to free form move attachments the way ON does it is a -
6) Sync has improved but is still slower than ON -
7) Beautiful interface is a + but with sections and pages shown too much screen space is taken vs Onenote for iPad a -
I think there are some features that ON doesn't provide and likely won't in the nearest future, that could really make Outline stand out & justify the added cost:
1) Ability to expand and collapse outlines, just like in desktop ON
2) Ability to search for tags, just like in desktop ON
3) Ability to export pages, sections or notebooks in HTML format preserving hierarchy structure (i.e. pages are separate HTML pages linked to the top section) and attachments. This would be a huge advantage that ON doesn't have and is unlikely to get anytime soon.
4) Ability to unlock all password protected sections that have the same password at once. I.e. somehow automatize sending this password to every section instead of doing it one by one.
5) Ability to select text & create an iOS Reminder from it.
6) Search and replace function