I think it would be a good idea if a deal is expired if its not something thats entirely obvious - such as a link that no longer exists or a price increase - then there should be a pop up appears to ask why, which could be added as the last post.
It could still be done anonymously - although I guess it would have to happen for every deal that was expired as it would be very difficult for the site to realise automatically that there was a price increase or changed conditions etc.
Its also seen it suggested a "are you sure you want to expire" pop up so if it is accidentally expired it will warn the person of their error (however, i think many people automatically click through "are you sure you want to do this" type messages.
However, if the pop up was looking for an explanation then it would be even more difficult to do accidentally, would require someone to lie or be pretty slow to mistake a deal as gone, and would keep mods time freer as it would reduce those expired needing corrected.
So what do you think - the powers that be might be listening!!!