I think it would depend upon other experiences that my game group has had.
I think that DF can be very detective-based, but can also be more of a sweeping saga of supernatural conflicts. Have the players seen...
* Buffy the Vampire Slayer (TV show, not the movie)
* Charmed (TV series)
* World of Darkness (RPG/novels/comic books)
* Kindred: The Embraced (TV knockoff of WOD)
* Dark Shadows (TV series)
* Diana Tregarde (books by Mercedes Lackey)
* Vampire books by Laurel K. Hamilton
* The Adept (books by Katherine Kurtz; Kerr? not sure)
* Constentine: Hellraiser (movie or comic books)
* Fred Saberhagen's Vlad Taltos books
* Kiolchak the Night Stalker (TV series from the 1970's)
* Bloodshadows (RPG from West End Games)
* Urban Arcana (d20 RPG from WotC)
* Indiana Jones (movies with a hint of magic behind the scenes)
* Harry Potter (books or movies)
For a detective style background, what about....
* Raymond Chandler's Philip Marlowe (books and some movies)
* Dashiel Hammett's The Maltese Falcon (book and movie)
* Ian Fleming's James Bond (the original books, not the movies)
For a better Harry Dresden experience, try...
* The Dresden Files TV series (even 1-2 episodes, just to start 'em off)
* There are several Dresden short stories, if they don't want full books.
* "Welcome to the Jungle" graphic novel (a fast read)
All of these can help to give a newcomer a feel as to how a modern-day supernatural game can be run. If they have seen this stuff it's easy to see how DF is played. There can be an emphasis on the wizards or vampires or fairies or whatever, and the Game Master can make his or her campaign follow one of many different styles. The Game Master can fit the game to what the players like or instead pick his/her own preferences.
So, what if they aren't familiar with any of this stuff? Explain that it's a modern-day world where magic can happen. Let them experience the setting a step at a time, with the players being amazed in the way that their characters would have been amazed in similar circumstances.
Hope that helps a little.