The Harry Potter line was one of the more successful licensing deals that LEGO has made.  In fact, I think it's probably second only to Star Wars, as far as longevity of the line of sets made.  Sets from the Harry Potter story were considered wonderful to many fans of the books, movies or both.  Even those who were not Harry Potter fans, loved many of the sets for their great assortments of parts.  Whenever a set is licensed and depicts a scene from a movie, it's much more likely to get new parts (from new molds; not just new colors or new printings).  Think of some of the many new parts that have come about from the Harry Potter line: We got a mermaid tail that's compatible with any normal minifig torso.  The heads of soft plastic for Dobby and the Gringott's Bank goblins let us make unique looking monsters.  It was the Harry Potter sets that introduced the owl, the baby dragon, the Hungarian Horntail, the dog and the standing rat to LEGO's lineup of animals.  The range of new parts from Harry Potter sets spans from the troll to the "snitch" 1X1 gold round plate to the feathered wing.

I do not believe that any of the aforementioned parts, however, are the greatest contribution of new parts from the Harry Potter line.  In order to fully appreciate a line of new parts, one has to look at their impact on a wider scale.  New parts such as hippogryph bodies may be highly unique, used by many builders and fetch a premium price, but they may not have nearly as deep of an impact on the whole population of LEGO fandom as other types of parts.  For example, Castle people will definitely find a use for those hippogryphs, but they aren't going to be used by many Space, Military or Town builders.  But practically everyone uses minifigs.  And while those minifigs will have wildly different torsos and handheld items across the spectrum of themes, all of them that aren't wearing a hat or helmet will have some kind of hair on their heads.

Almost a decade has passed since the first Harry Potter sets came onto the market, and I believe that all the new minifig hair pieces that came with them are the greatest contribution of new parts from that line.  My estimation of "greatest" comes from comparing broadness of use in the LEGO fan community, variety of parts produced and versatility throughout themes.  Take a look at the whole gamut of minifig hair pieces.  There are 32 different pieces (34 if you count different printings).  7 of the 32 originated from Harry Potter minifigs.  I'd say that 7 out of the 32 minifig hair pieces ever made is quite a big chunk for one theme to claim.

One of the best things about the hair pieces from Harry Potter sets is that LEGO has continued to produce all of them, with the exception of the Hagrid hair/beard piece, beyond the original line and used them in other sets.  When these hair pieces are used in other sets, they are often made in other colors which adds to their usefulness.  When the Hermione hair and Dumbledore hair first came out, they were some of the most sought after pieces LEGO had ever made.  (Of course, when those two pieces were introduced in 2002, they were 2 out of 16 hair pieces instead of 2 out of 32 like they are now.)  In the years since, we've seen the brown Hermione hair made in black, orange, reddish brown and clay orange.  The long light gray Dumbledore hair has been made in light bluish gray, black, dark red and tan.

You might be thinking that minifig hair doesn't really have as much impact as I've been giving it credit for.  If so, I ask you to consider what I believe is the most important trait the a hair piece bestows on a minifig: character.  Practically any minifig wearing the long Dumbledore hair will take on an appearance of age and wisdom.  The Gilderoy Lockhart hair gives the impression of an absent-minded or charismatic personality when worn.  Both the Ron Weasley and Harry Potter hairs confer a sense of youth and energy.  It's the wide array of characters and personalities that new hair pieces add so much to.  When you try to convey the personality of a minifig with a hat or helmet, you're really letting observers see the minifig for its occupation or duty.  This type of perception can usually be achieved just the same by the minifig's surroundings.  To create a minifig who is judged solely by his own constituent parts, you need to use the right hair.  The hair that sits atop a minifig's head in conjunction with his face often shows his character more than his surroundings, torso, posture or what he's holding in his hand.

The Harry Potter storyline is full of different characters with vastly different personalities.  Depicting these characters as minifigs had to be what led to the production of so many new hair pieces.  Set designers know, just as we AFOL's do, that making a minifig with a specific look that matches its character requires specific hair.  Try swapping hair on some Harry Potter minifigs to see what an impact it makes.  Putting Professor Umbridge hair on Hermione changes her look entirely.  Likewise, putting Gilderoy's hair on Voldemort makes him look disgruntled in a comical sort of way instead of menacing.  Even swapping Ron and Harry's hair makes them look completely different.  It's this ability to "make" the minifig with the hair and also the LEGO-inherent ability to swap hair pieces to make minifigs with entirely different characters that makes the new hair pieces in the Harry Potter sets so important.

Therefore, due to outstanding variety, continued production and one of the greatest potentials for enhancing minifig character to date, I give KUDOS to whoever designed the new hair pieces of the Harry Potter line.  Those 7 hair pieces have had a profound effect on minifig style, and I foresee them being used heavily for many years to come.

Now if I could only get my own hair to stop falling out... Stick out tongue