A couple ideas/suggestions...
Here are a few things to consider that might be a nice touch for versions in the future...
Promote a current free ad from the dashboard to a featured ad, some people find with a free ad doing well and getting it bumped down, having the option to upgrade that ad to a featured ad and reset the ad time/counter would be nice instead of re-doing it.
The ability to turn off the listings price option altogether for certain categories such as freebies and personals sections.
Customizable send out email for posting an ad as a registered member, "dear username you have posted an ad at blah blah and has been approved, click here to manage your listing".
A splash/landing page for categories, I don't know if there is a hack for wordpress that would do this, but something like a warning page/agreement for personals or descriptions for sales pages before you enter.
And a big one.... multiple locations selection/multi region from the one install, like melbourne.classifieds.com, sydney.classfieds.com etc etc. I'd be willing to pay more for that one.
