Here are a few items of feedback from my users about the classifieds:
1) Ability to require fields such as City, State, Country.
2) Add custom fields such as Color, Miles Driven, etc. About 5 custom fields is good.
3) Add custom checkbox items. Examples (for our car site) would be: Rebuilt engine, headers, leather seats, racing suspension, etc. Up to 10 would be good.
4) Add a custom introduction message to the top of the main page (easy to use via the admin panel).
5) Integration with vBulletin/vbPortal to make it look like the latest Photopost.
6) Better support for HTML-formatted ads.
By the way, the ability to comment on ads is an awesome addition. Thanks for putting that in!!