“They said of casting David Tennant as ‘Casanova,’ ‘D***, you should have cast someone sexier,'” Moffat explained at the Hay Festival (according to Radio Times). “With Benedict Cumberbatch, we were told the same thing: ‘You promised us a sexy Sherlock, not him.'”
At this point, remember, Tennant was a ways from his now huge stint as the Doctor, not to mention later roles like the lead in “Broadchurch.” And Cumberbatch was mostly known as a character actor who showed up in period pieces. Today, both are highly sought after and rather beloved by their fans.
It just goes to show that even the BBC doesn’t always know what’s right.