Number of Close/Delete votes needed should be a function of number of Upvotes/Answers/Favourites.
This would keep a balance between likers and dislikers. Prevents the situation where lots of people like a question and only a few are needed to vapourise it.
I'm sure there must be a downside to this idea, I just can't figure out what it is ; )
Update: Please see comments for the downsides pointed out by others.
Update #1: After reading Jeff's comments about 'mob rule' in this post, I should mention I feel the above idea should mitigate that problem. Because if you just say 'Over 400 upvotes means no deletion' you are favouring the upvoters. But by having the number of close/delete votes be a function of upvotes/answers/favs, you keep the same balance/ratio regardless of the questions popularity. Admittedly, I'm not exactly sure how this would play out with the 'Jon Skeet Facts' type extremes, but still worth a look I think.
Update #2: See Improved Question MergingImproved Question Merging SO blog post for info on how this was addressed.