Now d&d works for the "New rule" field, and I was wondering, whether it would be possible to make it simpler and enable drag and drop directly to the whitelist. 

Ideally I would imagine the following use case:
1. Alice finds an interesting website maintained by Bob. Bob uses Google Adsense to pay the bandwidth. 
2. Alice generally blocks ads, because she doesn't like all the flash popups and Adsense all over the place. However, at Bob's website adds are placed reasonably, and the topic of Bob's website suggests that Alice might actually use the adds.  
3. So Alice decides to let Bob's website display all the adds, irrespective of source. She drags and drops Bob's website from the location bar (or a bookmark to Bob's site) to the whitelist and all links blocked in this website become allowed for this website only.

Rationale - as the discussion on adblockers heats up, new ways of supporting certain "commercial" uses shall emerge. And probably Adblock Plus (for Firefox) has this feature.