vero - 8:25 pm on Jan 6, 2012 (gmt 0)
I noticed a big drop in CTR in December. I'd always used the default, and prior to December it had seemed to fit in with the rest of the site
When I looked at the site in December and this month, the font size for the ads seemed huge in comparison to the rest of the site (like Verdana medium vs Arial small), which made the whole site look rather schlocky (sorry - best description I would think of)
Not sure if this is related, but I'm going to specify the font size and see what happens.