There are a few features in aMSN that Adium should borrow.
1. Nudge support - Overall pretty useless, but a fun feature that other MSN IMes have that Adium is lacking.
2. Custom emoticons - the ability to use or see custom emoticons.
3. MSN 'Friendly Name' - The second(additional) line of text in an MSN user's name
4. Video Support
Now, I'm pretty sure that aMSN is open source, so couldn't Adium just examine the lines of code in aMSN and incorporate the useful ones? That would be a lot easier than writing them from scratch.